GTA
All Springer/NP/PCP Air Gun Discussion General => PCP/CO2/HPA Air Gun Gates "The Darkside" => Topic started by: Tomygun1 on June 24, 2022, 01:39:44 AM
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Please someone tell me why pellet velocity under 940fps seems to be the agreed upon speed limit for maximum accuracy in a 22cal pcp rifle. I’m not a newbie and I think we are missing something here. In a quality pcp or other air rifle it turns out that pellets can be just as stable as any projectile can be. I get the physics involved and it seems an artificial limit has been placed on pellet ammo thinking that pressure is an enemy of accuracy.
Looking for a flat trajectory my goal is longer range accuracy that is barely subsonic, around 1060fps at sea level, and I don’t care about foot pounds of energy. Optimum pellet weight and twist should sort out both speed and energy to do this while giving consistent accuracy. That only leaves pressure and it’s peak as a factor, and as a lifelong powder burner I know there is a sweet spot to be found regardless if it comes from stored air or burning gasses from gunpowder.
It’s my impression that a skirted pellet may have a speed limit, and if so then why aren’t we all shooting slugs now? I would like to hear success stories from others shooting high velocity.
Tom
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Pellets fired at over 1000 fps slow down very quickly.... An excellent RN pellet like an 18 gr. JSB Jumbo Heavy launched at 1060 fps it loses 67 fps in the first 10 yards, compared to only 44 fps starting at 940 fps.... Bump it up to 1180 fps, and it loses 98 fps in the first 10 yards.... In addition, the wind drift increases past about 850 fps, and quickly at over 1000 fps.... In a 10 mph crosswind, at 940 fps MV, it is 13.1" at 100 yards, at 1060 fps that increases to 14.14", and at 1180 fps to 15.65".... In fact that pellet drifts the least at 100 yards when the MV is only 855 fps....
In my experience, trajectory can be allowed for a lot easier than variations in the wind.... That is why you see virtually all the pellet shooters at the EBR shooting at just under 900 fps.... As to your question about why isn't everybody shooting slugs?.... The answer is, that if you are shooting at longer ranges (eg. 100 yards plus), you are handicapping yourself if you don't try and find a slug your gun likes.... as they have about half the wind drift of pellets.... and can be tolerant of velocities up to about 1050 fps or so without the drift increasing too much....
Bob
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Thanks Bob,
It sounds like we both are thinking that the ballistic coefficient is the firewall for pellets.
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Pellets have the worst BC next to round balls. So, if BC can be improved on by other factors than sectional density why aren’t we shooting long, hollow, balanced, and slender ‘pellets’ from a bbl with a faster twist? I remember the old bullets called “wasp waist” from the original Herters company that supposedly reduced bbl friction that would seem appropriate in low pressure air rifles.
Just contemplating.
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Tom,
Waisted pellets are aerodynamically stable. Shoot them too fast from a typical rifled airgun barrel, and their spin becomes excessive from 50 to 100+ yards down range; resulting in spiraling in flight. This is because pellets' forward velocity bleeds off very fast over range when you over drive them, while their spin speed does not.
The result is that gyro stabilization wants to keep the pellet flying with its axis pointing up (to counter drop, for long range shooting), while the aerodynamics try to force the long axis to stray tangent to the trajectory. So, gyro and aero forces interact with each other, resulting in spiraling. ( I am sure Bob can provide a more accurate explanation)
Bottom line is that if you want to shoot airgun projectiles "fast", use an appropriate slug. Else shoot whatever pellet weight you can drive at about 900 FPS. Certainly not over 950 FPS at the muzzle.
Videos demonstrate spiraling pellets, caused by overdriving them. Then slowing them down for stable flight to 100 yards:
If you want to jump directly to the most obvious spiraling, click here:
https://youtu.be/_Z3YNLBs8Lo?t=275 (https://youtu.be/_Z3YNLBs8Lo?t=275)
https://www.youtube.com/watch?v=_Z3YNLBs8Lo (https://www.youtube.com/watch?v=_Z3YNLBs8Lo)
The fix:
https://www.youtube.com/watch?v=QFou_4VqLBY (https://www.youtube.com/watch?v=QFou_4VqLBY)
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Pellet aerodynamic stability changes from stable to unstable as the speed approaches Mach 1. Since the pellet has gyroscopic stability as well, it still remains gyroscopically stable. The start of the problem with high speeds is that if the pellet is fired at a very high speed, say close to 1100ft/sec it may be aerodynamically unstable to start and then become aerodynamically stable as the speed drops. This means that at some point it will be neither stable nor unstable, it is what we call neutral stability. This is bad as there will be infinite gyroscopic stability at that point and very high values either side, which will give increases in group size. In addition, as the speed drops further the dynamic stability of the pellet becomes worse so any yaw there may be will increase in magnitude leading to spiralling.
Despite what has been claimed on videos, spiralling is not due to gyroscopic overstability and the pellet trying to follow the trajectory, the gyroscopic stability is simply not high enough once the pellet has slowed past the neutral stability point, and the angle of the trajectory is not high enough. I have seen the results of gyroscopic overstability, and it is nothing like spiralling.
If you really want to shoot at very high speeds you will want a low twist rate barrel to try to delay the effects of dynamic instability increases, and a heavy pellet to reduce the velocity drop, though it will always be high with pellets. In addition, you will have to have a very good match between the pellet and the barrel to reduce any pellet yaw or yaw rate on leaving the barrel so that the dynamic instability has nothing or very little to work on. Any crosswind present will still be able to induce some spiralling.
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These posts pretty well explained the velocity limitations of pellets. We just don’t currently have the ability to keep a pellet/slug supersonic all the way to the target….so we keep them well subsonic to avoid that drag spike that happens at trans sonic speeds (and it happens to all projectiles).
Never got any kind of decent accuracy at trans sonic speeds. Did “cheat” and get good (not quite great) accuracy with one rifle at +1020fps speeds….the cheat was that for the first 25 yards, was shooting though a barn...so it was wind protected and by the time wind could get to the pellet, it had slowed down significantly. Take the same rifle/ammo/speed out into the field, where the wind could get to it from the get-go, and accuracy was much more disappointing.
Generally, with airguns, once the chronograph starts reading over 925-950fps, I’ll look for something heavier/slower.
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Thanks Miles, but I am confused:
If reducing the barrel twist rate helps with pellet stability when fired at high muzzle velocity, then how is it a lack of gyro stability that drives spiraling? My pea-brain seems to think the quoted texts below contradict each other.
...the gyroscopic stability is simply not high enough once the pellet has slowed past the neutral stability point...
If you really want to shoot at very high speeds you will want a low twist rate barrel
If reducing the barrel twist rate reduces spiraling, then the pellet was spinning too fast when fired at high muzzle velocity. As spiraling does not happen until pellet velocity has decayed, and shooting the same pellet at a muzzle velocity similar to the decayed velocity of the fast one at 100 yards, does not producing spiraling, what is left seems to be over spinning pellets. Spin speed being dependent on muzzle velocity and barrel twist rate. Reducing the velocity with a fast twist barrel has a similar effect on pellet spin speed, as shooting it fast from a slow twist barrel.
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It is not a lack of gyroscopic stability, what I meant was that once you are below the speed at which there is neutral aerodynamic stability or close to it, i.e. the pellet is some way down range, there is insufficient spin to produce the degree of gyroscopic overstability needed to cause major problems. For it to be a gyroscopic overstability problem at normal pellet speeds i.e. below about 1000ft/sec though the speed will depend on the pellet design, you need spin rates about 5-10 times faster to give significant flight problems.
There is sufficient spin as the pellet slows down to increase the dynamic instability problems which will give a steady increase in peak yaw and from that spiralling which using a slow twist rate barrel can delay.
Hope that helps.
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Thanks, Miles
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We've had this slug/pellet/spin rate discussion several times...each time something comes up that makes me rethink the whole thing.
Started by taking away all spin with a true smooth bore.
Pellets are kind of shape-stable. The center of mass is well ahead of length center. Accuracy will be terrible, but with a real smooth bore, pellets with deep hollow bases and heavy round nosed heads, still hit nose first at 40-50 yards. We spin them to get accuracy, evening out imperfections and the little “nudges” of wind. Already kind of predisposed to land nose first. Lots of tail drag, weight forward, thinking we tend to over spin pellets.
Not too sure about Miles view.
Drag slows the pellet down pretty quickly...rotational speed/spin does not slow down much at all...so at ranges where the pellet slows down significantly, it’s still spinning at nearly the same rate as it started, so the relationship of twist rate, actual spin rate to velocity changes drastically. Towards “over spun” rather than “under spun”.
At some point, they just go stupid (spiral or just plain tumble).
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I would like opinions/thoughts from some of you in the following thread since the same science is applicable:
https://www.gatewaytoairguns.org/GTA/index.php?topic=200579 (https://www.gatewaytoairguns.org/GTA/index.php?topic=200579)
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Pellets have the worst BC next to round balls. So, if BC can be improved on by other factors than sectional density why aren’t we shooting long, hollow, balanced, and slender ‘pellets’ from a bbl with a faster twist? I remember the old bullets called “wasp waist” from the original Herters company that supposedly reduced bbl friction that would seem appropriate in low pressure air rifles.
Just contemplating.
They’re called slugs, and if you shoot them out of the right barrel, you can get really nice down range velocities and accuracy. Use the right tool for the job.
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I fully agree with the facts here but RMAC winner this year shot 26gr 22 pellets at 970fps and Ted was shooting 33.95 .25 pellets at 935fps and both shot great obviously.
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I fully agree with the facts here but RMAC winner this year shot 26gr 22 pellets at 970fps and Ted was shooting 33.95 .25 pellets at 935fps and both shot great obviously.
It is all a matter of getting the perfect launch for your pellet from the gun barrel. If there is no yaw or yaw rate as the projectile leaves the gun, then anything can be made to give small groups. The above velocities can be used with slow twist rates and optimised projectile launch to get good results depending on local conditions. The worst problems will occur at speeds above Mach 0.95 where the stability aerodynamics begin to get bad.
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The accuracy of h&n terminators from my D48 .177 at 1060 fps has been bending my mind lately. Anybody considering the physics of a pointy spike leading the pellet's way at sonic speeds?
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It seems different barrels do better with standard speeds (880-900) while other barrels do well with higher speeds as Troy said above. I know my .22 RAW's non choked poly barrel is more accurate shooting 18-25gr peleets @ 940-980fps (slugs also) than @ 880-900fps. Just need to tune ones PCP to it's liking! Enjoy reading all the great threads here, a lot of incredible info!
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I...is more accurate...
At what range? If you shoot at 50 yards, then you may not know or care what a projectile does at 100 yards. That is a perfectly legitimate usage model.
Shooting pellets "fast" to be effective at 100 yards is where spiraling may show up.
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It seems there are several opinions here about twist speed and velocity. The longer the projectile is the faster the twist needs to be in order to stabilize it but once stable velocity no longer matters.
1) a projectile fired @1000fps from a bbl with an 18” twist at 100yds will spin 600 times during its flight.
2) a projectile fired @2000fps from a bbl with an 18” twist at 100yds will spin 600 times during its flight.
The faster projectile arrives in half the time and is subject to half the gravity and wind drift. The rpm’s don’t change within the 100yd distance as in both cases there have been 600 revolutions but the faster one has completed it in half the time of flight.
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It seems there are several opinions here about twist speed and velocity. The longer the projectile is the faster the twist needs to be in order to stabilize it but once stable velocity no longer matters.
1) a projectile fired @1000fps from a bbl with an 18” twist at 100yds will spin 600 times during its flight.
2) a projectile fired @2000fps from a bbl with an 18” twist at 100yds will spin 600 times during its flight.
The faster projectile arrives in half the time and is subject to half the gravity and wind drift. The rpm’s don’t change within the 100yd distance as in both cases there have been 600 revolutions but the faster one has completed it in half the time of flight.
wrong... rpm is different... times it spins at distance is almost same...plus way to simplified... different projectiles slow down at different rates...rate of spin slows at a way slower rate than velocity...
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Tom
The faster projectile loses forward speed more quickly than the slower one, so the faster projectile's time of flight is longer than half that of the slower one. As such, the faster projectile spins more times to 100 yards than the slower one (spin decays much more slowly than forward speed)
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I...is more accurate...
At what range? If you shoot at 50 yards, then you may not know or care what a projectile does at 100 yards. That is a perfectly legitimate usage model.
Shooting pellets "fast" to be effective at 100 yards is where spiraling may show up.
Have taken ground squirrels @ 118yds with 18gr pellets, so, yeah they're not spiraling @ 965fps because it was in altitude.....not a guess.....guys have shot 18gr pellets @ 1050fps accurately.....maybe your gun is not accurate with higher speeds doesn't mean others are not. Martin Rutterford builds .22 HM1000x LRT's to shoot JSB 25gr @ 950fps his tune out of the box for 100yd competition, I believe he knows what he is doing....
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Wind drift is proportional to LAG TIME, not flight time.... The lag time is the difference between the actual flight time and what it would be in a vacuum (ie no drag).... The faster projectile has more drag, and hence a greater lag time, and therefore more wind drift.... Here is a chart for a pellet at 50 yards....
(https://hosting.photobucket.com/albums/oo221/rsterne/Ballistics/.highres/Wind Drift JSB 50_zpsio6vbeef.jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/ad32680a-7969-443d-ab0e-5a65c7b8979e/p/74635217-23d0-490d-87bf-f15cfe4b398b)
and here is a similar chart for bullets at 200 yards....
(https://hosting.photobucket.com/albums/oo221/rsterne/Ballistics/.highres/Wind Drift G1_zpsix0tzr8y.jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/ad32680a-7969-443d-ab0e-5a65c7b8979e/p/bac0babb-d1a1-465f-b320-b9aa179eb5f7)
Note you have to push a bullet at 2500-3000 fps to bring the wind drift back down to what it was at 900 fps.... :o .... We are very fortunate with airguns to be able to operate in the "sweet spot" of velocities right around 900 fps.... 8)
Bob
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Thanks for clearing that up, Randall
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I was just comparing in another post on another forum what GTA is like and how it compares to other forums.
This thread illustrates one of the major differences to AGN: a lot of who I consider my GTA heroes have participated in this complicated but important ballistic topic.
And it has been enlightening, helpful, civil even humble ("pea brain"), informed, deep. 👍🏼
Keep it up, gents. Discussions like this are awesome! 😊
Thanks for taking the time to enrich all of us. You're definitely part of the hard core that makes GTA what it is.
And the other part of "the core" is the whole huge section of springer specialists and modders — again, where else can you find so much expertise?!? 😊
Matthias
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Bob,
I am grasping your explanation of lag time and if I get it it’s the difference of shooting in a vacuum or through the atmosphere, velocity reduction. I haven’t heard it called that before. I still have questions though and I am aware that frontal pressure increases with additional velocity at an exponential rate but tapers off below 2000fps. Still I expect 25% reduction from the ‘extra speed’ from a .036bc pellet but that still leaves a 75%gain from the extra 100fps @1050fps. If that’s the case the pellet should arrive at 100yds with less time of flight, less drop (gravity), and less exposure to side wind (drift). These would be desirable features for both accuracy and range.
A remedy for all might be a longer, hollow, pointed, skirtless pellet. Yeah, I know the reasons pellets have always been skirted but I don’t know why in these modern rifles we cant have higher bc pellets (not to be confused with slugs). A longer probe would overcome the seating and engraving problem and friction would be minimal, probably a 1/14” twist or so. What do you think?
Tom
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As I said, time of flight is not the determining factor in wind drift, it is the lag time.... There is an excellent explanation of wind drift, how and why it occurs, at Reply #35 in this thread.... https://www.gatewaytoairguns.org/GTA/index.php?topic=169459.20 (https://www.gatewaytoairguns.org/GTA/index.php?topic=169459.20)
In flight, the pellet does not actually experience a "side wind" at all, it turns to face into the apparent wind (the vector sum of its forward velocity and the crosswind), which leaves a drag component in the downwind direction, which causes the drift.... Since the drag in the Transonic range (Mach 0.8-1.2) is hugely greater than below that, the wind drift increases, so a Transonic pellet actually drifts more.... It will however, have a flatter trajectory, because that DOES depend on time of flight....
Here is a chart showing the rapid increase in drag coefficient vs. velocity, for several shapes of projectiles (drag models).... GA is typically used for pellets, and G1 for slugs (and they are nearly identical)....
(https://hosting.photobucket.com/albums/oo221/rsterne/Ballistics/.highres/Drag Coefficients Including ChairGun_zpskfyhpxvr.jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/ad32680a-7969-443d-ab0e-5a65c7b8979e/p/fb175467-5019-43c7-94aa-a1897bd7ce71)
Note what happens between 900 fps and 1350 fps (Mach 0.8-1.2), which is called the Transonic Region.... Also note that the drag in the Supersonic never drops back to its Subsonic values.... Refer to the charts in my previous post to see the effect of muzzle velocity on wind drift....
Bob
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Thanks Bob,
I appreciate your explanation and my desire is to learn, and if I have it wrong I want to know. I have many charts also. Your ‘lag’ time doesn’t make any sense to me as it is hypothetical assuming no lag time occurs only in a vacuum. It seems what you are calling lag time in the atmosphere in reality is the reduction of velocity due to frontal pressure and drag, not to be confused with drift which is a sideward motion. While sectional density will always remain the most important factor when it comes to resisting drift and frontal pressure there's no getting around the fact that the longer any projectile remains subject to wind drift the greater the effect will be. It is noticeable at 100yds, but the proof in the pudding is that wind drift for a projectile at 1000yds is greatly affected by time of flight, thus velocity. Sure, we are talking pellets at subsonic velocities but the physics don’t change, and their horrible bc’s make them among the worst offenders with a lengthly time of flight. Honestly, I’m not looking for a disagreement but want to know if the physics of what I have been taught is wrong. Please allow me to respectfully disagree.
Thanks,
Tom
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sorry but when it feels the wind is the biggest factor... the longer it is on the new path the further it will hit from the poa... it is simple geometry...
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The bullet does not get pushed sideways... it turns nose first into the wind...
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https://www.ssusa.org/content/what-exactly-is-wind-drift/ (https://www.ssusa.org/content/what-exactly-is-wind-drift/)
https://www.ssusa.org/articles/2016/12/7/expert-forum-wind-shooting (https://www.ssusa.org/articles/2016/12/7/expert-forum-wind-shooting)
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The bullet does not get pushed sideways... it turns nose first into the wind...
actually meant to say... it turns nose first into following the wind... concentration probs do not always type/proof read what I think I did...
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You can disagree if you like, many do, because they don't understand the science, and it is admittedly counter-intuitive.... There are lots of articles on this, by ballistics experts like Brian Litz, and a good one may be found at https://kestrelinstruments.com/mwdownloads/download/link/id/100/ which contains the quote below.... Take the time to educate yourself, I don't have time to argue after I have done my best to explain....
Background Theory
McCoy’s Modern Exterior Ballistics provides a solid discussion on computing how the wind
affects a bullet’s flight, with the punch line being that the amount of deflection, D, a bullet
experiences from a wind, W, is proportional to the ‘lag time,’ or the difference between the
bullet’s time of flight in a vacuum (the distance to target, R, divided by the muzzle velocity, MV),
and the actual time of flight, T.
Equation:
D = [T - (R / MV)] x W
Good luck!....
Bob
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I'll add a bit to Bob's explanation to maybe help, but he is absolutely right on this - both on the causation, and that if you don't understand it correctly that it is a misapplication of the relevant physics . . . .
I'll start by saying that the nomenclature - "wind drift" - is itself misleading: while "wind" is indeed the source of the forces that induce the drift, it is not the wind (or as generally viewed, the movement of the air between the muzzle and the target pushing on the side of the pellet) that leads to the movement of the projectile. It is the impact of the drag forces that the wind causes.
What actually happens when a projectile is shot with a side wind is different than people think. The overwhelming force in the equation is momentum, and that makes the pellet try to fly straight to the target. But the side wind leads to a situation where the pellet turns into the apparent wind (see Bob's explanation) and this direction is different than the vector that the projectile is actually flying on - you could think of it as "skidding" a bit in the air. It is these drag forces that define the amount of wind drift (by pushing the path of the pellet off the initial line of momentum to the target), and since the drag forces go up as the pellet crosses into the transonic region, the drift goes up as the speed goes up even though the time in flight goes down. It is counter intuitive, but that is only because intuition is wrong in this case - where "intuition" is based on basic physics and not the specific physics of ballistics. It is not a case where someone's understanding of physics is "wrong" but more one of being "incomplete."
Do also note that side wind can also lead to a drop or even a climb of the pellet relative to POA too, not just the lateral shift we commonly think of. There is a lot going on with the drag forces involved.
Hope that helps . . . .
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Good explanation, Alan.... The vertical deflection is called "Aerodynamic Jump", and causes groups shot in gusty conditions to form a diagonal line on the target, rather than horizontal.... Incidently, pellets do the opposite deflection in a crosswind, because the CP is behind the CG instead of in front of it!.... :o
(https://hosting.photobucket.com/albums/oo221/rsterne/Ballistics/.highres/Pellet Drift Chart 12 FPE 3 mph 25m_zpsikrbnsvd.jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/ad32680a-7969-443d-ab0e-5a65c7b8979e/p/f7e5c0ae-cd4b-4dc2-af7e-75ac6f699869)
This chart (for pellets, at 25m in 3 mph wind) is from Mike Niksch, maker of the Thomas BR rifles.... Note the 3 and 9 o'clock positions to clearly see the Aerodynamic Jump.... Also note the drastic change in POI when the wind is shifting from 11 to 1 o'clock, or 5 to 7 o'clock....
Bob
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also notice the on the chart following vs opposing wind effect... opposing wind longer flight time... following wind shorter flight time...
then you mix in whether it is right or left hand twist... usually right hand/clockwise...
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if your range is kept 'practical' like under 60 and you have serious furball incapacitation as your main goal and not look at me i can shoot da farthest type nonsense, then about 1050 is a sweet sweet spot to keep that point blank killzone about flat all the way ... its all about compromises though and what your focus is imo ..
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if your range is kept 'practical' like under 60 and you have serious furball incapacitation as your main goal and not look at me i can shoot da farthest type nonsense, then about 1050 is a sweet sweet spot to keep that point blank killzone about flat all the way ... its all about compromises though and what your focus is imo ..
Crosman Destroyers at 1100 fps muzzle velocity make a mess of small pests at close ranges. Keep it close and have fun. Going long? Slow it down to the 900 range and save yourself some grief. I'm not a math so that's how I keep it simple.
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True for all forms of shooting (fire/air/archer/even spear throwing I guess).
Are things we can know and control with nearly a certainty…velocity, projectile weight, gravity.
Things we can know within a specific range….projectile BC. Often are not the published figures in your air gun, it does vary a bit even between shots. Really should self-test them and believe YOUR numbers.
One thing we can’t get to live within limits, is perpetually an invisible variable...the wind. We can adjust the above known items to our advantage, but the wind always wins.
It’s not constant over any real distance, changes speed, direction, swirls around, suddenly shifts, and just can’t be constant over 50 yards.
Even worse in field type hunting.
Picture your self in the middle of Bob’s “wind circle” above. Able to shoot 360 degrees and the impact changes.
There is still an “art” in shooting , it’s not all purely thoery/graph numbers….the ability to out guess the wind more times than not.
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The bottom line is.... "Gravity is predictable, the wind is not".... Shooting a projectile faster than it's "sweet spot" for minimum wind drift just doesn't make sense.... For pellets that is around 850-950 fps, while for slugs it is likely higher, about 900-1050 fps.... This latter information is because typical airgun slugs do not match the existing drag profiles very well, and appear to have less drag in that velocity range than the G1 profile used to calculate their BC.... Hence they have less wind drift in that wind range than the G1 model would predict.... If Miles Morris new drag profile is correct, here is the effect that lower drag in the 900-1050 fps velocity range has on wind drift....
(https://hosting.photobucket.com/images/oo221/rsterne/Wind_Drift_Miles(1).jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/ad32680a-7969-443d-ab0e-5a65c7b8979e/p/791a1163-57ea-4463-8c61-c5f145bec8e9)
Note, however, that once you approach Supersonic, even with the lower drag suggested by the new profile, the drift increases rapidly at Mach 1....
Bob
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"Gravity is predictable, the wind is not."
Bob
Bob, I think I'm going to have that framed to hang on the wall of my shooting corner.
Great statement!
Matthias
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Think we’ve beaten the idea into submission...sub sonic, high BC, seems to be the way to go with airguns.
Even if you launched supersonic, it won’t last much distance, and the pellet will just go though Bob’s graphs from right to left.
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Wind drift is proportional to LAG TIME, not flight time.... The lag time is the difference between the actual flight time and what it would be in a vacuum (ie no drag).... The faster projectile has more drag, and hence a greater lag time, and therefore more wind drift.... Here is a chart for a pellet at 50 yards....
(https://hosting.photobucket.com/albums/oo221/rsterne/Ballistics/.highres/Wind Drift JSB 50_zpsio6vbeef.jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/ad32680a-7969-443d-ab0e-5a65c7b8979e/p/74635217-23d0-490d-87bf-f15cfe4b398b)
and here is a similar chart for bullets at 200 yards....
(https://hosting.photobucket.com/albums/oo221/rsterne/Ballistics/.highres/Wind Drift G1_zpsix0tzr8y.jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/ad32680a-7969-443d-ab0e-5a65c7b8979e/p/bac0babb-d1a1-465f-b320-b9aa179eb5f7)
Note you have to push a bullet at 2500-3000 fps to bring the wind drift back down to what it was at 900 fps.... :o .... We are very fortunate with airguns to be able to operate in the "sweet spot" of velocities right around 900 fps.... 8)
Bob
Regarding wind drift at 200 yds, it's interesting that the bottom of the subsonic curve moves right as BC decreases in the chart. Thinking of pellets, this could be the difference between a lower BC pellet having increasing drift begin at 1050 fps and a higher BC pellet having increasing drift begin at 950.
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It is a function of the total wind drift being a result of the average velocity, and a lower BC slows down more, so it has to start out faster to have the same average velocity.... A projectile with a BC of 0.05 would not likely be shot at 200 yards, but I included it to show what happens.... Note that the pellet chart, which is only over 50 yards, is shifted to a lower MV for the same reason, it's average velocity is higher (and therefore the sweet spot for MV lower) with only 50 yards to travel, instead of 200.... If you charted a pellet over 100 yards, the "sweet spot" would move to a slightly higher velocity, but not as much as you might think, because of the sudden loss of velocity (and high drag and drift) near the muzzle when the MV is high....
Bob
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Thanks Bob! The explanation with 50 vs 200 yd charts is clear and interesting to me.
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Yes, for any projectile, there is one specific velocity where the wind drift is the least.... The optimum MV is where the projectile spends the most time close to that velocity, over the range you are interested in.... It's actually not too hard to find that with most Ballistics Calculator, you input the BC, and the range to target, and then keep changing the MV to produce those charts.... As the range increases, or the BC decreases, the "sweet spot" for the MV rises, relative to the velocity with the least drift....
In practical terms, you are better off to hedge on the higher velocity side of the curve (eg. say 900 fps on the 50 yd. pellet chart), because the total drift is not much greater, and the trajectory is flatter.... I strive for 950 fps for most RN pellet shooting where the ranges are up to 100 yards....
Bob
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Rsterne,
Thanks for your patience and the clear and concise presentation of data. It reminds us that our intuition about physics often needs a reset when things are more complicated that we'd previously believed.
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To the point that some guns are accurate at high speeds, I'll offer this ten shot group of my .22 Air Ranger shooting JSB 18.1s at 1020 fps at 55 yards. It was a “dead calm” day (note – in reality there is no such thing), leading to a fairly tight group. As good as it is, it is no where near as good as other groups shot on another very calm day, but shot at about 900 fps – that photo is unlabeled for speed, and I think the “one that got away” was from a small breeze that I did not notice. Both of these groups were shot holding dead on the same POA and not doping for any wind, as I was trying to shoot with no wind those days. But that is hard to do, as there is almost always some level of air movement out there, whether we see it or not.
In the OP, it was asked why we all aren't shooting slugs right now. The truth is that I prefer pellets as I shoot almost exclusively at home, and the 55 yards is about the limit for me - I can get to 65, but that is shooting from the front to the back and I prefer to keep it all in the back yard. And slugs just don't give me any advantage in my shooting - and cost more too. But I am experimenting some with them . . . .
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I’m just curious guys if anyone has it in them to add a little more. How does your spin rate decline at a slower rate than your velocity? I know your rpm goes up and down as velocity rises and falls, but isn’t the projectile still spinning the same amount as it was when it left the muzzle? For example a 1:10” twist is still making one revolution every ten inches no matter the speed, or I assume it is. Seems like there would be a direct correlation with speed and rpm. Y’all have forgotten more than I know about ballistics and I believe that y’all are right but I can’t figure it out.
Beau
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rpm is a unit of time not distance... pellets lose velocity quickly so more time to distance... nearly at same rpm it left muzzle with...
http://bulletin.accurateshooter.com/2008/06/calculating-bullet-rpm-spin-rates-and-stability/#:~:text=Bullet%20RPM%20is%20a%20function,when%20it%20leaves%20the%20muzzle. (http://bulletin.accurateshooter.com/2008/06/calculating-bullet-rpm-spin-rates-and-stability/#:~:text=Bullet%20RPM%20is%20a%20function,when%20it%20leaves%20the%20muzzle.)
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I’m just curious guys if anyone has it in them to add a little more. How does your spin rate decline at a slower rate than your velocity? I know your rpm goes up and down as velocity rises and falls, but isn’t the projectile still spinning the same amount as it was when it left the muzzle? For example a 1:10” twist is still making one revolution every ten inches no matter the speed, or I assume it is. Seems like there would be a direct correlation with speed and rpm. Y’all have forgotten more than I know about ballistics and I believe that y’all are right but I can’t figure it out.
Beau
You are correct that at the muzzle, there is a direct correlation between pellet spin rate and pellet velocity. However, as the pellet flies down range, there is no fixed relation between the two. The pellet velocity is lost through aerodynamic drag, a relatively large force which quickly reduces the velocity. The pellet spin is also reduced by the aerodynamics, in this case the spin damping moment, which is a torque produced by the air friction around the pellet body. The spin damping reduces the pellet spin but is usually a very small aerodynamic effect which can only reduce the spin rate fairly slowly.
The result is that nearly all spinning projectiles lose forward velocity faster rate than the spin rate decrease as the pellet flies down range.
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Gotcha! So a 1:10 twist at the muzzle is turning 1 turn in 10” but at 50 yards may be doing 1 turn in 8”? (Just an example no math)
That cleared it up perfectly, it’s very simple don’t know why I couldn’t figure that out.
Thanks Miles
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For a pellet, that is probably a close estimate....
Bob
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A 1000fps .200 BC projectile in a 1:10 barrel should be about 1:9.5 at 50yds.
And about 1:8 at 270yds.
All assuming insignificant rate of spin decay.
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Thanks guys I appreciate it. As said earlier in the thread, this is what puts GTA over the top. I love AGN and I’m on there more than GTA because of the volume of traffic, but it’s a quality over quantity thing. They’ve got the quantity but the quality is right here. Y’all are awesome!
Beau
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These are values measured by Harry Fuller years ago on a JSB 25.4 grain .25 pellet fired at 850ft/sec. The twist rates are in inches per turn.
2 feet = 1: 18.9
225 feet/ 75 yards = 1:15.9
300 feet/ 100 yards = 1:14.9
450 feet/ 150 yards = 1:13.2
600 feet/ 200 yards = 1:12.8
From these figures, I derived the following spin damping values based on the European/Modern US definition.
Mach Clp
0.00 -0.0188
0.60 -0.0175
0.80 -0.0165
1.00 -0.0149
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I want to thank you all for the knowledge, advice, and experience shared here, and clearly you have answered my initial questions and then some. While as an old shortimer I may still be able to show you how to put your bullets on the bullseye at 1000yds, you are helping me now to put my pellets on the bullseye at 100yds consistently. I’m successful at 75yds but that last 25yds still evades me and I just can’t quit. I think the best experiment about exterior ballistics was done by the British Army in the 70’s with the flight of a 105mm SPBT projectile filmed in ultra slow-mo against a gridded background, from the muzzle to impact.
It overcame tremendous yaw to finally travel true and left a lasting impression on me regarding the importance of the shape of a projectile, and if that old film could be found it might answer many questions that still continue.
Thanks,
Tom
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......... Regarding wind drift at 200 yds, it's interesting that the bottom of the subsonic curve moves right as BC decreases in the chart. Thinking of pellets, this could be the difference between a lower BC pellet having increasing drift begin at 1050 fps and a higher BC pellet having increasing drift begin at 950.
OK, I'm convinced by all this that there is a velocity sweet-spot for wind resistance. (Or whatever we should call it.) And you can't argue against Bob's point that bullet drop is constant but the wind is not. Most shooters want increased velocity so as to flatten trajectory, until the point of diminishing returns, and I guess this is that point, moreso than just turbulence near mach one.
But since the "sweet spot" for drag is different for each projectile shape, weight, BC, etc, how are we supposed to figure out the optimum fps for a particular projectile? Is there some kind of partial (or comprehensive) list somewhere?
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Fortunately, the curve of drift vs velocity is pretty flat through the range of velocities from 800-950 fps for the typical RN pellets we would use when concerned about longer ranges (where drift is more important).... My rule of thumb is to use the highest velocity where your pellet is accurate, staying below about 950 fps.... For a lot of JSBs, that occurs around 880 fps, but I have found little difference up to 950 in many of my guns, so that is my aim.... providing the accuracy is there....
For slugs, the same thing applies, up to about 1050 fps, accuracy is king....
One other consideration is that the closer you push the velocity to the speed of sound, the worse the efficiency will be.... ie shot count will drop.... It has nothing to do with accuracy, but it certainly is of practical importance....
Bob
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One other consideration is that the closer you push the velocity to the speed of sound, the worse the efficiency will be.... ie shot count will drop.... It has nothing to do with accuracy, but it certainly is of practical importance....
Bob
So on this point is there any reason it’s less efficient, other than it obviously takes more air to push a projectile faster? The way you wrote it makes me think something else is going on.
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If this sends there should be a pdf chart of ballistic coefficients for commercial pellets attached.
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The basic reason is that it takes more air to drive a pellet faster, because you need to keep the valve open longer.... Once the valve is open past the time when the pellet travels more than half the barrel length, you use a lot more air and gain only a small amount of velocity.... There are other effects as well, such as the possibility of sonic choking in the valve throat, which when it occurs produces a 53% drop in pressure across the choke point.... In addition, the mass of the air charge released to accelerate the projectile becomes as large, or even greater than, the weight of the projectile, so more potential FPE goes into moving the air than the projectile....
As an extreme example, I have a 6mm PCP that will produce 173 FPE (1030 fps) with a 73.4 gr. slug at 4500 psi, with an efficiency of 0.55 FPE/CI (the valve is basically open until the pellet exits the muzzle).... I shot a 1.8 gr. airsoft BB through it, same tune, and it hit 2092 fps (17.5 FPE) which is only 1/10th the efficiency.... The air mass used per shot was several times the weight of the plastic BB.... :o
By using Helium, which is 1/7 the density of air, and has a higher speed of sound, you can achieve much higher velocities.... but with air, using huge amounts more air to get supersonic, just to lose that velocity within a few yards of the muzzle.... makes no sense....
Bob
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Yes, for any projectile, there is one specific velocity where the wind drift is the least.... The optimum MV is where the projectile spends the most time close to that velocity, over the range you are interested in.... It's actually not too hard to find that with most Ballistics Calculators .....
So to be clear, BC (assuming it's calculated correctly) plus fps is all that's needed to find the wind-factor sweet spot?
And what about velocity loss over distance? Do most ballistic calculators take this into account, changing BC over time, etc?
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Ballistics Calculators take into account the velocity loss over distance.... However, they assume a constant BC, which means you have to use the Drag Model that most closely matches your projectile.... Realistically, we will use G1 (or some calculators offer GA, which is nearly the same), and one calculator (Easy BC) can use Miles' new drag model for slugs....
You input your BC, and the range you need, the crosswind, and then vary the MV and watch the drift change at that range.... After a few tries, you can narrow it down to find the optimum MV for that BC and that range....
Bob
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Strelok Pro ballistic calculator gives you two more options:
▪ Use up to 5 different BC numbers — for different velocities (this will account for the GA drag model and others to not fit your pellet perfectly).
▪ Use a custom drag model — where you can add as many cd values (drag) — each related to a velocity (in mach).
I'd love to research a custom drag model for some of the hollow points we're shooting!
Matthias 😊
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I'd love to research a custom drag model for some of the hollow points we're shooting!
All it takes is time, a LabRadar (or two Chrony's that read identically), and an adjustable PCP (oh, and LOTS of ammo).... The range of velocities covered by the drag model will depend on the variety of velocities that you shoot.... As an example, with a LabRadar you can track a 25 cal slug to about 70-75 yards, so if it starts at 900 and drops to 850, then you have to shoot another set (at least 10 shots each) starting at 850, then 800, then 750, then 700, then 650, then 600, then 550, then 500.... Then of course, you need to test them at 950, 1000, 1050, 1100, 1150, 1200, etc.etc. for as far as you want to have your model work.... Then you have to put all the data into a spreadsheet, calculate the Drag Coefficients for each velocity range, and hope you can draw a fair curve through all the Drag Coefficients.... Assuming you can, then you will have a drag model for that slug....
Bob
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Yes, Bob, I'm lining up the required resources, it's been taking me a couple of years of saving up for it....
A Labradar wasn't in the budget, but two chronos with LED. Once the gun is set up, I will post a thread to get some advice from the pro's before making too many mistakes.... ;)
Below are two HP pellets that show a lot of promise for good hollow point expansion.
With a custom drag curve (or a velocity-adjusted BC) shooters could predict how far they can shoot it with a certain gun/ power and still get expansion.
Matthias
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I'd love to research a custom drag model for some of the hollow points we're shooting!
All it takes is time, a LabRadar (or two Chrony's that read identically), and an adjustable PCP (oh, and LOTS of ammo).... The range of velocities covered by the drag model will depend on the variety of velocities that you shoot.... As an example, with a LabRadar you can track a 25 cal slug to about 70-75 yards, so if it starts at 900 and drops to 850, then you have to shoot another set (at least 10 shots each) starting at 850, then 800, then 750, then 700, then 650, then 600, then 550, then 500.... Then of course, you need to test them at 950, 1000, 1050, 1100, 1150, 1200, etc.etc. for as far as you want to have your model work.... Then you have to put all the data into a spreadsheet, calculate the Drag Coefficients for each velocity range, and hope you can draw a fair curve through all the Drag Coefficients.... Assuming you can, then you will have a drag model for that slug....
Bob
If you use the above method, you really need to do it three times on different days to take into account different weather conditions.
The other way to do it is to create a drag curve based on the shape of the pellet and then fire much fewer pellets at selected speeds and compare the measured results with the drag curve predicted results. In this way, you can adjust (fiddle) the estimated curve to match the measured velocity drops using much less ammunition. This is pretty much the standard way for large calibre stuff, where ammunition is expensive.
Of course, the tricky bit is creating the drag curve.
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Disillusioned.
(pre-radar units...was using two chronographs….and a check, would reshoot swapping the near and far chronographs)
Testing for BC, found the same pellets can give widely different BC’s when tested in different rifles. DIY testing will give differences in BC (even between shots), but some times would find .01 or more from published BC numbers. Oddly, a widely different than published BC didn’t predict accuracy (you’d think the “off” rifle would shoot poorly).
Pellets are not the constant. Can work up data for a rifle that’s useful (rifle being the constant), but not for a pellet.
Got to wondering why, but without lab type equipment, can’t be sure. Suspect the “off” rifles lost velocity early from “extra” pellet motions that were dampened out as range increased. Testing the BC at longer ranges (starting at 25 yards and ending at 50yards) “evened up” the BC difference considerably from starting at 5 yards.
Disillusioned, started to just treat each PCP and an individual, not trusting the published pellet BC figures to be correct for every air gun. If it’s accurate at range, will just make a drop chart of various ranges. About 75-85% of the time it’s in line with the published figures.
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I agree with Dean that everything is best done with one gun - BC does vary by gun . . .
I don't have two Chronys, but I have tested BC by shooting two test samples of 30 pellets over the Chrony at the muzzle and at a distance of 21 yards (my limit indoors, so that wind is not a factor). In addition to getting the data needed to calculate the BC for that pellet in that gun at that muzzle speed, I also learned something very important that held in every test I ever did like this: the ES is significantly higher down range than at the muzzle, and this is despite the fact that I worked very hard to keep the ES tight at the muzzle (topping up air every few shots while shooting in the middle of the seet spot to begin with).
In thinking through this, there really can only be one thing that I can think of that would yield this: pellet to pellet variation in BC. This likely is a function of minor differences in the shape or weight of the pellet going into the barrel, and the resulting shape after being shot. For example, differences in head size will lead to different lengths of the the front "flat" from engagement with the rifling. This variation, thought of as a relative change in the shape of the pellet, will be far higher for a pellet than for a traditional bullet.
Anyways, this variation is likely to be a huge factor in accuracy as it will also lead to different performance both in terms of drop and wind drift at a given distance, even if the ES at the muzzle is very tight . . .
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I have experienced situations where the ES is actually smaller downrange.... This would agree with theory, for different weight pellets, as the heavier ones (which should start slower) should have a slightly higher BC, and lose a bit less velocity, tightening up the ES.... This does not, however, always occur, but it can....
Unfortunately, it only takes a few fps difference in the velocity loss to make a huge difference in the calculated BC.... so the whole process is fraught with errors.... The relatively new "Easy BC" calculator will report both the lowest and highest BC, as well as the average, if you input the percentage ES for both near and far Chrony readings.... You will be shocked at the range of BC values.... ::)
Bob
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Bob,
I agree on the theory, and I was expecting a lower ES at the target but that was not what I found. I ran 4 or 5 of the tests using two different guns and a couple of good pellets in each, and in each case the ES was between 1.5 and 3 times higher at the target . . . .
In the end I gave up trying to really nail down the BCs, but mostly because I decided it was not really worth it without a lab radar - working of the averages was not compelling enough for me to continue. It did leave me with the thinking that the pellet to pellet variation in BC - as shot - was a significant factor that we simply could not control for without sorting pellets. In fact, it caused me to be believe that the big thing we gain with sorting is probably a reduction in that BC variation, assuming we sort for both head size and weight. And of course those are always where I find the best groups . . . as much as I hate sorting, it does work!
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You guys are amazing!! Thank you all for sharing. That's why I come here.
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Responses to the topic of just why we limit velocity of pellets for long range accuracy have been great. And while testing and mathematical reasons have shown how the BC of pellets change downrange, like all other projectiles, I haven’t yet heard anyone mention the importance of beginning with the highest bc pellet at the muzzle, including the highest velocity that should extend the practical range of a pellet rifle. Paper math aside, it’s hard to deny that if you begin with a BC advantage from the muzzle that it should tend to continue. If it doesn’t then the forces downrange are affecting a less than perfectly shaped projectile.
I can only speculate on the proper pellet shape or velocity needed. What is the length, weight, sectional density, grove/land fit, pressure, hollow depth, point and tail configuration, bbl twist, with or without choke, teflon lube, and overall design that will end up working the best. What I do know is that as refined as it is all the present pellet technology has going for it is 75yrs. old, and old smooth bore shotguns did the same with this pellet design without rifling until rifled slugs appeared to improve them.
I hope to hear from modern shooters that expect greater range and velocity from these PCP pellet rifles that should be capable of so much more.
The whole point of this original post was to find out what modern shooters are doing to extend their accurate pellet ranges beyond the accepted velocities of 880fps. There just must be more!…
Tom
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Isn’t this why slugs have become popular?
May still have the velocity/drag limits, but doesn’t the higher BC to start with extend the range?
Isn’t this also why heavier/higher BC pellets have become popular as well?
Can think of several airguns (mostly discontinued) that were freakishly fast and yet got reviews that claimed “excellent” accuracy. Perhaps not shot at any great distance, but high velocity does have it’s fans.
This one from 2015 had the heaviest .22 pellets of the time clocking 1115fps….and the lightest at 1357fps. Fascinating at the time.
Eun Jin Sumatra 2500 air rifle test review (hardairmagazine.com)
But as the state of air gun projectiles now stands….can’t really beat the speed of sound all the way to a long range target. It’s going to have to go through that transition “hump” around the speed of sound.
That “hump” doesn’t actually predict an accuracy problem...does predict both a “disturbance” and a windage problem that would make for an accuracy problem.
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Here is pretty much state of the art for a High Velocity small bore... Cedric did a .17 HMaiR Mrod a few years ago... Here is the Mark II version...warning a couple graphic Shots at end of vid...
https://www.youtube.com/watch?v=mTLB9iK3h9E (https://www.youtube.com/watch?v=mTLB9iK3h9E)
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Here is a Quick vs of,
Red = 15g .172 with a .066 B.C. may be a bit generous since is at 1500 fps...
Green = 15g .172 w/.08 bc at 1050 fps ( just to compare same slug at lower fps)
Blue = 24g .172 w/.126 bc at 1050 fps heavier slug at same fps
maroon 30g BBT w/.08 bc at 1050 fps just cause wanted to show how it compares with the .172 15g at same speed...
I asked Bob to draw up a light BBT (in .25 and .22) in 2014 or so... the 30g BBT .22 and 40g BBT .25 is what resulted...
I knew that they would have advantages in wind and downrange fpe..
But...
My main reason was for the independence of it... as long as you could get accuracy at near stock power levels it would still be worth doing at even down at 650-800 fps... would get ya dinner at 40 -55 yards...
So point is the traditional pellet is a very good projectile as is and has it's strengths/advantages... so it is with slugs at thier best...
But even at less than optimum as long as you can find accuracy tis still worth doing... just different tools for different jobs/uses...
below is a Gamo Urban sending the 30g BBT about 650 fps at 60 yards...
https://www.youtube.com/watch?v=Oer-gOyXDLc&t=4s (https://www.youtube.com/watch?v=Oer-gOyXDLc&t=4s)
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Thanks for the video plug, Kirby! Yes sir, my Urban loved the BBTs and my new Cayden does too. It's sending them downrange just over 700 fps in stock form. I'll try to do another can bust with it for comparison. I do have to say that BBTs don't seem to care how slow you shoot them. My Prod pushes them around 400 and they fly straight from it too.
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Ok, so many have expressed the reasons they believe that for accuracy's sake the velocity must be limited, and many charts and studies have been presented to explain why they believe a pellet will be most stable below 900fps when encountering the atmosphere.[size=78%] But it seems it should be based on the physics of a projectile in flight bucking a side wind when we are discussing drift caused by a side wind. Physics shows that the projectile will face into a side wind and tend to point in that direction and correctly so. The side mass is greater at the rear so why would it do anything else. Proper projectiles with a high BC are all narrower at the front than at the rear and subsequently a side wind affects the rear more than the front and should point the front into the wind. But the whole mass will tend to follow the effects on its exterior and move in the direction of the atmosphere. Does anyone not take the wind into consideration when making a long range shot? Clearly our BC technology with pcp pellet rifles can get better and we shouldn’t rest on knowledge from the past.[/size]
[/size]
[/size][size=78%]The question still remains; To reduce time of flight and the time the wind has to affect side travel why aren't we striving for higher velocity. To recap, we are only talking about long range shooting.[/size]
[/size]
[/size][size=78%]Thanks again for your interest and input.[/size]
[/size]
[/size][size=78%]Tom[/size]
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You need to read some articles on ballistics, and how and why the wind causes a projectile to drift.... It is the lag time that is critical, and that increases dramatically in the transonic region (Mach 0.8-1.2) because of the huge increase in drag.... This is not just a theory, but has rock solid data from thousands of ballistics test to back it up.... It may be counter-intuitive to you, but those are the facts....
Bob
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hey Bob where did the Sonic choking thing end up..? some of the convo got past my concentration level...had my little theory... and only understood some arguments... but have stuff to look up to understand it all...
so...dropped out...
was consensus ever reached..?
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Thanks Bob,
And I am on the same page as you. I understand the published studies on lag time that you refer to. The topic is why aren’t we producing pellets that have a correct sectional density/weight/ and BC to travel at 1040fps at sea level better than we are now. It may turn out that it requires a longer 20cal projectile with a very rapid rifling twist to get the best results, I don’t know. With firearms 26cal is the magic number for maximum velocity/sectional density. But I do know that all calibers in pellet guns can be improved on over what we now accept. What I do know is that since I was 13yrs old pellet performance has only slightly improved. I am an octogenarian and back then it was also thought that an airplane couldn’t exceed the sound ‘barrier’. A lifetime in shooting sports hasn't left me uninformed and where we seem to differ is only with lag time. It is my position that lag time is overcome with velocity and improved BC and that seems to be being ignored and has created a ‘barrier’ keeping air rifle velocities slow. Clearly its not a problem in supersonic velocities but we are talking subsonic speeds, and like with high velocity there is a point of diminishing returns not to be exceeded. It’s my position that we aren’t yet close to the ‘sweet spot’.
Thanks for your knowledge,
Tom
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"It may turn out that it requires a longer 20cal projectile with a very rapid rifling twist to get the best results, I don’t know."
No... any projectile with CG in front and high drag at the rear needs treated like a dart...less twist will always be better...
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Twist will always be related to the length of the projectile. At the velocity we are discussing I’m not convinced that a boat tail design is the answer but it couldn’t hurt. CG in front and drag at the rear is the enemy.
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We have those they are slugs... we/ Bob Started doing them long ago... we also have little pointy slugs flat base and cup base... The boat tail Are called Bobs Boat Tail...
and yes and no to your enemy statement... there are benefits to pellets and in the last five years heavy pellets have become more prevalent... things have evolved...
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and a long dart will need less twist than slug...
even at subsonic a boat tail has advantages...
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yes .257 is a bit of a sweet spot for slugs with air power...
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Thanks Kirby,
I get the slug thing and shoot them well and pellets also. But try to understand just how limited pellets in our modern pcp rifles are these days due to their horrible sectional density and BC. Length and weight distribution will be a factor in improving these projectiles and no one will disagree that the longer the projectile is the faster it must spin to stabilize. Not the same principle as with a skirted narrow wasted non aerodynamic blunt pellet. Still not talking slugs, a longer slim hollow pellet spinning faster can only perform better at the same weight than the lead darts we call pellets that we now shoot.
I hope that you, like me, want to improve on what we have come to accept as the best it can be. What we have works great at short range.
Tom
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Pellets require very little spin to be accurate, start thinking about 100 calibers per revolution, up to 200.... That means 22-44" twist rates for a .22 cal, and pellet length has little to do with it, as pellets are flare stabilized.... There is an excellent article on this in this thread.... https://www.gatewaytoairguns.org/GTA/index.php?topic=169459.0 (https://www.gatewaytoairguns.org/GTA/index.php?topic=169459.0)
Pellets behave in a similar way to darts, but do require a slow spin to even out imperfections in their construction.... Pellet weights, and hence their SD, have increased greatly in the last decade.... A heavy .22 cal 15 years ago was 21 gr., now they go all the way up to 34 gr., a 60% increase in SD.... I was one of the first to experiment with slower twist, I designed a 26" twist barrel in .30 cal which won the EBR the first year it was used.... Since then, FX in particular have led the way in slower twist barrels for pellets, although the slowest are still found in benchrest rifles like the Thomas BR, which can shoot 25.4 gr. pellets at 100 yards into dime-sized groups....
Bob
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Length and weight distribution will be a factor in improving these projectiles and no one will disagree that the longer the projectile is the faster it must spin to stabilize. Not the same principle as with a skirted narrow wasted non aerodynamic blunt pellet. Still not talking slugs, a longer slim hollow pellet spinning faster can only perform better at the same weight than the lead darts we call pellets that we now shoot.
I hope that you, like me, want to improve on what we have come to accept as the best it can be. What we have works great at short range.
Tom
It is not as simple as a longer projectile needing a higher or lower spin, particularly something which uses aerogyro stabilization. The optimum barrel twist rate will depend on the size of the aerodynamic stabilizing moment and how much supplementary gyroscopic stabilization is needed to minimize group sizes. If the long projectile has much larger aerodynamic stabilization moments, then a lower twist rate may be needed, but this will also depend on the ratio of the axial and transverse moments of inertia. The barrel twist rate will also have to keep clear of the projectile's resonant frequencies, normally being kept at a higher twist rate above the resonant frequency rather than lower. Both the moments of inertia and the resonant frequencies will depend on the mass distribution of the projectile as well as the aerodynamics.
You may well also find that a longer projectile, which depends on flare stabilization, will have a higher drag and thus a lower BC due to the air seeing more of the flare at small yaw angles.
As for cross wind response, thinking in BCs and velocity is not really helpful, you really need to think in terms of aerodynamic drag coefficients and how they grow as speed approaches the speed of sound (1116.5 ft/sec). If the correct reference drag law is being used, then the BC should not change or change by very little as the speed increases, which tends to hide from people's thoughts the increase in the drag coefficient. Thus, thinking about BC and velocity alone will not give you a true picture. The fact is that, particularly for a blunt nosed projectile be it slug or pellet, the drag will increase. There comes a point where the drag increase more than offsets the use of a higher velocity, giving an increase in the lag time and thus the down wind drift.
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As for cross wind response, thinking in BCs and velocity is not really helpful, you really need to think in terms of aerodynamic drag coefficients and how they grow as speed approaches the speed of sound (1116.5 ft/sec). If the correct reference drag law is being used, then the BC should not change or change by very little as the speed increases, which tends to hide from people's thoughts the increase in the drag coefficient. Thus, thinking about BC and velocity alone will not give you a true picture. The fact is that, particularly for a blunt nosed projectile be it slug or pellet, the drag will increase. There comes a point where the drag increase more than offsets the use of a higher velocity, giving an increase in the lag time and thus the down wind drift.
Miles,
How does this relate to SD?
I'm thinking specifically of the difference between a 34 grain 22 cal slug, and a 34 grain 25 caliber slug, both with the same nose and tail shape.
Obviously, the 22 cal will be longer / thinner. Based on past discussions, it seems that the 22 cal will have better BC, and thus might be a better choice for long-range hunting, if a flatter trajectory is preferred.
- but what about lag time / wind resistance?
(And of course, barrel design may also play a major part in this, but I'm hoping that there's a reasonable theoretical answer when using a "typical" modern pcp rifle. )
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...and higher drag of the .25 is what creates it having more lag time... if that's what you are asking...
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If the Form Factor is similar, then the higher SD of the .22 cal compared to a .25 cal of the same weight will results in a better BC.... The equation for BC is....
BC = SD / FF
and a .22 cal will have a 29% higher SD than a .25 cal of the same weight....
Bob
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/
The equation for BC is....
BC = SD / FF
and a .22 cal will have a 29% higher SD than a .25 cal of the same weight....
I really didn't like math in school.
If my teachers would just have done math as Bob does it — I would have had much better grades — Bob makes it fun!
Lovin' the simplicity of a formula — that spells out complicated relationships of aerodynamics, physics, the works. Just plug in the numbers, and —— out comes the result. 👍🏼
Fascinating.
Matthias
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what about in practical terms
.22 wins on BC which needs distance to take advantage of, wind drift goes to the .22
efficieny to the .25
is there an example how much % more energy would you get on .25 with the same gun same weight projectile?
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If the Form Factor is similar, then the higher SD of the .22 cal compared to a .25 cal of the same weight will results in a better BC.... The equation for BC is....
BC = SD / FF
and a .22 cal will have a 29% higher SD than a .25 cal of the same weight....
But what about barrel design?
If someone made a 177 cal rifle specifically designed in every way to optimally shoot a 50 grain 177 cal HP slug, that would probably be a great hunting gun. (Assuming the slugs expanded enough upon impact.)
- But that rifle doesn't exist. Nor does one optimally tuned for a 34 grain 22 cal, and I don't expect to see one anytime soon.
And if I've understood most of the above, the 34 gr 22 cal & 25 cal projectiles will require different twist rates, for optimum "immunity" to wind.
So given the "typical" twist rates, chokes, and barrel lengths, (yeah, I know....) which of these slugs has the best chance of long-range accuracy in the field?
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Roughly speaking, with the same weight pellet in the same gun, but changing from .22 to .25 cal has the POTENTIAL to increase the power (FPE) by about 29%, but only if the porting is full bore size in both cases.... If you optimize the gun in .22 cal and then just change to a .25 cal, about half that increase (14-15 %) is typical.... It may use a bit more air to produce that, but generally the efficiency of the larger caliber (in FPE/CI) will be slightly greater....
The 25.4 gr. JSB Monster in .22 cal and 34 gr. Monster in .25 cal have reputations as shooting well.... The 34 gr. Beast in .22 cal has been challenging to get the best accuracy from, but I'm sure the right barrel will eventually be found.... Presently I don't know what that might be....
Bob
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Hypothetically speaking lets look at the possibilities inherent in a 177cal pcp rifle projectile as proposed by Allan. A hollow bullet shaped non skirted ‘pellet’ around the weight he suggests would have a length of about .5” and a tremendous SD (sectional density) compared to the pellets we now shoot, and SD in a projectile when stabilized by a proper rifling twist can overcome a lot to increase the practical range of accuracy and retained energy. The smaller diameter 177cal punches a smaller hole in the air with less frontal resistance, the longer projectile is subject to greater side affects from wind, while the greater SD and BC struggles to overcome that. I can see a net gain here.
I am reminded of the studies done by the US military after adopting the .556cal over the previous.308 ammo. They were so pleased with the results that it led to the development of the ‘needle gun’. As a field weapon it was a failure but the principle was sound and it worked. Stabilize a long thin projectile with a high SD. The pressure used in our pcp rifles and the reasons we use them might be reason enough to take a second look at this technology to increase practical range and accuracy. I’m not looking to resurrect the needle gun, but lessons learned apply to increasing range and retained energy with good SD numbers with pellet rifles.
Hypothetically.
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Without digging too deep into "the science" of high velocity accuracy, I will say at shorter ranges accuracy can be had at 1000+ fps. This probably has been covered already, apologies for back stepping if so. But I've found 30 to 45 yards (if you're lucky) to be about the limits for pellets, slugs can go at least double that at 1000fps+. There are a few 177 pcp's capable of over 30fpe and slugs can be pushed fairly quickly in comparison to pellets. If you choose a regulated pcp, make sure you have a big plenum or potential (peak) power will suffer.
Another thing to consider, is lead fouling. Every airgun that I've pushed velocities into the 1000+fps range, the barrels lead up badly. I would think a slower twist brass barrel (or barrel liner) would be the way to go, but it's just a thought.
Although not shot from a pcp (But a springer, mod 135), here's a recent group I've had at 25 yards pushing .177 10.5gr piranhas at 1005-1010fps. Four went into the cluster with one outlier. Haven't had the chance to push the distance further.
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"If someone made a 177 cal rifle specifically designed in every way to optimally shoot a 50 grain 177 cal HP slug"
hmmm good luck getting it up to speed... it would be needing ~150 fpe Cedric is at around 100 fpe with his .172 and that is with a 27+ inch barrel...
second... twist rate... would be high possibly causing a large amount of yaw/jump...
now if you take the wasp waist of the Herters Bullet or more like the 34g .22 monster but subtle...with some flare at base for drag and bearing surface...
then give it a drag inducing b.c. killing 5g or so hollow base.... may get it to work in the 9 twist that .172 barrels tend to come with...
so impose the above onto this nose/body...
http://www.geoffrey-kolbe.com/cgi-bin/drag_working.cgi?unit_length=inches&weight_unit=grains&bullet_name=.338+air&re_calculate=yes&boundary_layer=L%2FT&entry=return&diameter=.182&length=.9&nose=.450&meplat=.04&drive_band=.182&base_diameter=.182&angle=0&boat_tail=0&secant_radius=&weight=52.6&density=11.4 (http://www.geoffrey-kolbe.com/cgi-bin/drag_working.cgi?unit_length=inches&weight_unit=grains&bullet_name=.338+air&re_calculate=yes&boundary_layer=L%2FT&entry=return&diameter=.182&length=.9&nose=.450&meplat=.04&drive_band=.182&base_diameter=.182&angle=0&boat_tail=0&secant_radius=&weight=52.6&density=11.4)
or with a bigger meplat
http://www.geoffrey-kolbe.com/cgi-bin/drag_working.cgi?unit_length=inches&weight_unit=grains&bullet_name=.338+air&re_calculate=yes&boundary_layer=L%2FT&entry=return&diameter=.182&length=.9&nose=.450&meplat=.08&drive_band=.182&base_diameter=.182&angle=0&boat_tail=0&secant_radius=&weight=55.5&density=11.4 (http://www.geoffrey-kolbe.com/cgi-bin/drag_working.cgi?unit_length=inches&weight_unit=grains&bullet_name=.338+air&re_calculate=yes&boundary_layer=L%2FT&entry=return&diameter=.182&length=.9&nose=.450&meplat=.08&drive_band=.182&base_diameter=.182&angle=0&boat_tail=0&secant_radius=&weight=55.5&density=11.4)
here is one with a lower b.c. because of the blunter nose...
http://www.geoffrey-kolbe.com/cgi-bin/drag_working.cgi?unit_length=inches&weight_unit=grains&bullet_name=.338+air&re_calculate=yes&boundary_layer=L%2FT&entry=return&diameter=.182&length=.79&nose=.182&meplat=.08&drive_band=.182&base_diameter=.182&angle=0&boat_tail=0&secant_radius=&weight=54.3&density=11.4 (http://www.geoffrey-kolbe.com/cgi-bin/drag_working.cgi?unit_length=inches&weight_unit=grains&bullet_name=.338+air&re_calculate=yes&boundary_layer=L%2FT&entry=return&diameter=.182&length=.79&nose=.182&meplat=.08&drive_band=.182&base_diameter=.182&angle=0&boat_tail=0&secant_radius=&weight=54.3&density=11.4)
but we are talking a hollow base bullet... skirt-less is what defines that...
gonna need a lot of helium to even get close to the fpe needed tho... n prob a bench gun only... chances of it working right ::) ::) ???
https://www.google.com/search?q=herters+wasp+waist+bullet&rlz=1C1GNAM_enUS688US688&source=lnms&tbm=isch&sa=X&ved=2ahUKEwiAu4WNvoT5AhWeD0QIHSUhChwQ_AUoAnoECAEQBA&biw=1094&bih=474&dpr=1.25 (https://www.google.com/search?q=herters+wasp+waist+bullet&rlz=1C1GNAM_enUS688US688&source=lnms&tbm=isch&sa=X&ved=2ahUKEwiAu4WNvoT5AhWeD0QIHSUhChwQ_AUoAnoECAEQBA&biw=1094&bih=474&dpr=1.25)
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You cannot keep on just increasing the length of a projectile and thinking you can use more twist rate to stabilize it. Yes, you can get gyroscopic stability, but you will run into severe problems with dynamic stability, which is already pretty bad with many small calibre projectiles. In artillery projectiles, the maximum length diameter ratio which can be given acceptable stability is between six and seven calibres. For small arms it is considerably less, but I do not know the exact values.
There also seems to be some confusion on wind effects. Spin will not affect the down wind drift, it will have some effect on the second order vertical effect, which many shooters claim to have never seen. Projectile length will again have no effect on down wind drift. The only effect it will have in regard to wind drift is to give a very slight increase in skin friction drag which in theory will change the drift but not by any amount you are ever going to see.
As for using a shape similar to the Monster pellet, you would be better going for a slug type shape with small bore riding driving rings combined with the correct barrel twist rate. The flare at the tail of the Monster is a very crude way of changing the aerodynamic stability of the projectile and produces a lot of unwanted drag. Similarly, the head shape is not very good as speeds approach Mach 1.
I would use caution when using the Kolbe calculator for projectiles with meplats larger than about 15%. If it uses the McDrag equations, they are not applicable for larger meplats.
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Hypothetically speaking lets look at the possibilities inherent in a 177cal pcp rifle projectile as proposed by Allan. A hollow bullet shaped non skirted ‘pellet’ around the weight he suggests would have a length of about .5” and a tremendous SD (sectional density) compared to the pellets we now shoot, and SD in a projectile when stabilized by a proper rifling twist can overcome a lot to increase the practical range of accuracy and retained energy. The smaller diameter 177cal punches a smaller hole in the air with less frontal resistance, the longer projectile is subject to greater side affects from wind, while the greater SD and BC struggles to overcome that. I can see a net gain here.
I just have to say that five pages into this topic, I am disappointed to read the bold underlined highlighted text . . . .
The side area of the pellet has no bearing on the wind drift, and the immediate statement right after it about the hypothetical improved SD and BC "struggling" to overcome the increased side area is just wrong. It is all about the lag time caused by the drag, so the BC is basically everything in this equation, with the side area having no impact (as a discrete factor, assuming it can be stabilized to begin with) . . . .
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Roughly speaking, with the same weight pellet in the same gun, but changing from .22 to .25 cal has the POTENTIAL to increase the power (FPE) by about 29%, but only if the porting is full bore size in both cases.... If you optimize the gun in .22 cal and then just change to a .25 cal, about half that increase (14-15 %) is typical.... It may use a bit more air to produce that, but generally the efficiency of the larger caliber (in FPE/CI) will be slightly greater....
The 25.4 gr. JSB Monster in .22 cal and 34 gr. Monster in .25 cal have reputations as shooting well.... The 34 gr. Beast in .22 cal has been challenging to get the best accuracy from, but I'm sure the right barrel will eventually be found.... Presently I don't know what that might be....
Bob
i thought fpe was proportional to caliber times smallest port diameter
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darn physics...always getting in the way of what you wish could be.
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I agree, Alan.... Some people are just stubborn in their beliefs, and won't bother to read the literature.... No amount of trying to help them works.... I have seen this too many times to count.... The idea that side area has anything to do with wind drift is one of the most common, and the concept of lag time (which increases drastically in the transonic) is too counter-intuitive for many to believe.... As you say, 5 pages of trying to help, just doesn't....
Baco, you are correct, I use caliber times smallest port size instead of caliber squared (times PI/4 in both cases) when calculating the area that the pressure works on to get the force on the projectile.... Therefore, if you keep the same port size, but increase the caliber, the force goes up roughly in proportion to the caliber, and also the potential FPE for a given pressure and barrel length.... Likewise, if you leave the caliber alone and increase the port size (up to bore size), the potential FPE rises roughly in proportion to the smallest port diameter (pressure and length fixed)....
I can't supply you with the math to support the use of caliber x smallest port diameter instead of bore^2, but it seems to be a close enough approximation when dealing with so many variables, and 50% or greater loss for actual FPE over theroetical (mostly because of air mass and friction).... The maximum possible FPE (assuming full port area) is still Max FPE = Area (sq.in) times Pressure (psi) times Barrel Length (ft.).... Dividing that by two is a good "lofty goal", seldom reached in practice....
Bob
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Roughly speaking, with the same weight pellet in the same gun, but changing from .22 to .25 cal has the POTENTIAL to increase the power (FPE) by about 29%, but only if the porting is full bore size in both cases...
Say what? Why would fpe go up, if the projectile weight and speed are the same? Newton's second law of motion, no? Obviously there would be some difference in the size of the wound, and also a difference in penetration (inversely proportional) but FPE?
I'm not arguing with you, (I'm not quite that foolish) but this seems pretty counter-intuitive.
Anyway, again, I'm asking about wind resistance, as relates to SD and twist rate. (And maybe choke design.)
If Miles is correct, then I guess it doesn't matter, but others here have posted the exact opposite.
My head's starting to hurt... :-X :D
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In that hypothetical, the speed is not the same - presumably the "tune" or settings on the gun would be the same before and after, thus the power (and thus speed of pellet) would go up, if going from .22 to .25. One could then "down tune" the gun to the same speed and see increased efficiency and shot count . . .
Given the number of different hypotheticals in this thread, it is not hard to get confused . . . . ;)
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Say what? Why would fpe go up, if the projectile weight and speed are the same? Newton's second law of motion, no? Obviously there would be some difference in the size of the wound, and also a difference in penetration (inversely proportional) but FPE?
I'm not arguing with you, (I'm not quite that foolish) but this seems pretty counter-intuitive. "
here we are talkin interior ballistics... and at the same barrel length... and fpe at the muzzle... the 34g .22 has about 1/3 less area to push against to accelerate the round... the .25 34g needs the bigger porting to be able to supply that 1/3 more air...
the higher the sd the harder to accelerate the round in the barrel...
now give the .22 about 1/3 more barrel length and it evens out a lot.. and the amount of air to get the fpe a closer match...
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Allan.... Pretty simple, the base area of the projectile in a larger caliber provides a bigger surface for the pressure to work on, so the force is greater.... Force = Area x Pressure.... Energy = Force x Distance (which is at a maximum the barrel length)....
Sectional Density works exactly the opposite inside the barrel and in flight.... It helps maintain muzzle energy downrange, but it makes the projectile harder to accelerate in the first place....
Bob
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"
Anyway, again, I'm asking about wind resistance, as relates to SD and twist rate. (And maybe choke design.)
If Miles is correct, then I guess it doesn't matter, but others here have posted the exact opposite.
My head's starting to hurt... :-X :D "
this is now exterior ballistics... the 34g .22 has about 1/3 less area to induce Drag than the lighter for caliber 34g .25 has...
think pie are squared.. Pi X radius2
so .22 has an area of .038 squared inch
and .25 has an area of .049087 squared inch
so they both have the same weight but the wider .25 has more drag... that drag is the cause of the lag time... so the .22 has less drag that causes drift...
twist rate well in general as sd goes up twist rate needs to go up... but can be mitigated by aerodynamic stability...
also high twist rates can have their challenges and effects... the faster it spins the more perfectly balanced it needs be...
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You pretty much have it, Kirby.... 8)
Bob
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More mass in the rear? I don't know. The head generally speaking, is the same dia. however the domed nose is also solid. More surface in the skirt, more weight (mass) perhaps in the nose. A couple of dynamic forces at work. Figuring this out seems more complex then at first glance to me.
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I'll try one more time to explain how and why the Lag Time (and hence the wind drift) does not decrease with velocity.... Here is the appropriate chart, which I have just derived using ChairGun....
(https://hosting.photobucket.com/images/oo221/rsterne/Lag_Time_Etc.jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/ad32680a-7969-443d-ab0e-5a65c7b8979e/p/b23a5367-8fe3-4251-85ba-08ea748c04fe)
The distance is 100 yards, and the pellet is the 18.1 gr. JSB Heavy, with a BC (GA) of 0.036.... The orange curve is the GA drag model, I only have the numeric values of that up to Mach 2, but the entire model is built into ChairGun.... The Time of Flight in a Vacuum (blue line) is simply the distance (300 ft.) divided by the Muzzle Velocity (fps).... The Actual Flight Time (green line) is calculated by ChairGun.... The Lag Time (black line) is the Actual Time, minus the Time in a Vacuum, and for ease of viewing I multiplied the actual difference by 5.... The wind drift in a 10 mph crosswind, calculated by ChairGun, is at a minimum of 12.5" at Mach 0.8 (~900 fps), and increases to 18.4" at Mach 1.5 (~1700 fps) before starting to decrease.... It does not drop back to 12.5" until Mach 2.7 (~3000 fps)....
What I hope this chart makes clear, is that the drastic increase in drag in the Transonic region (Mach 0.8-1.2) causes the Lag Time to increase, even though the Time of Flight in a Vacuum is decreasing.... The minimum wind drift occurs at Mach 0.8 (~900 fps), shooting faster than that increases the drift....
Bob
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..... so they both have the same weight but the wider .25 has more drag... that drag is the cause of the lag time... so the .22 has less drag that causes drift...
twist rate well in general as sd goes up twist rate needs to go up... but can be mitigated by aerodynamic stability...
also high twist rates can have their challenges and effects... the faster it spins the more perfectly balanced it needs be...
So other than wound diameter, the 22 cal seems to have it all over the 25 cal, assuming your barrel has enough twist rate.
I'm still fighting the idea of investing in a 22 cal rifle, but logic seems to be screaming at me to do it.
If one was shopping for a 22, with the specific intent of shooting heavy slugs at a little under 1,000 fps, what kind of twist rate might be optimal for a barrel in the 19" - 22" range?
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..... so they both have the same weight but the wider .25 has more drag... that drag is the cause of the lag time... so the .22 has less drag that causes drift...
twist rate well in general as sd goes up twist rate needs to go up... but can be mitigated by aerodynamic stability...
also high twist rates can have their challenges and effects... the faster it spins the more perfectly balanced it needs be...
So other than wound diameter, the 22 cal seems to have it all over the 25 cal, assuming your barrel has enough twist rate.
I'm still fighting the idea of investing in a 22 cal rifle, but logic seems to be screaming at me to do it.
If one was shopping for a 22, with the specific intent of shooting heavy slugs at a little under 1,000 fps, what kind of twist rate might be optimal for a barrel in the 19" - 22" range?
I would go for a .224 barrel (222 rem, 223 rem). I have 24" 12.7 twist barrel and it shoots 47 grainers at 980 fps with 200 bar.
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You would need to know how heavy, and therefore how long.... I would suggest you pick out something from the NSA range of slugs, and then ask that question to Nick.... I am sure he will have an answer for you.... If you are looking at something around 40 gr., then you probably can't go wrong with the 16" twist of a .22LR barrel.... If you are purchasing a barrel from TJ's, they have 14, 16 and 20" twists, but they are 0.222" or 0.224" groove diameter, so larger than a .22 pellet barrel.... I have a 14" twist TJ's in .224 cal, and it will handle slugs up to 47 gr....
Bob
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look I mainly just shoot the BBT 30g and just tested the K.O. 25g (and then bought a few more tin's) an have a couple of H&N .217 samplers...I hand pump so 50 fpe and less is what what I do...
My .22s n slugs...
Max Maximus believe to be 20 twist... 2000 psi 26" barrel
45 fpe with 30g BBT , 50fpe with .25g K.O.
sends K.O. best followed by 30g BBT...
as expected the 225-37g Noe not accurate and started to be unstable at 35 or so yards... the 223-38 did not fare any better...
in general around 30g is about tops for 20 twist... and up to 40g should be using 16 twist...
the lee 225- 55-rf comes in two flavors one a short round nose called the Bator that drops at 50g... and the other a more pointy nose drops at 55g..
those will probably work in the 14 twist of the .22 hornet barrel I have... in fact at about 45 grains should be 14 twist... imo...
but Knifemaker has got a round in the 38-39 grain weight range to work very well in a 1 : 20 twist sleeved Redman's .22 lr liner...and could not get it to work in the 16 twist barrel he had even though that is the twist that is usually best for that weight/length round... so sometimes things that are not supposed to work do...
My AT 44L is at 44 fpe and is very accurate with the BBT 30g in the 220/222 size sized to .217... developed a bad leak have not fixed yet so no testing the K.O. or H&N slugs yet... Pretty sure 17.7 twist like L.W. barrels
Both my Buccaneer(mid 30 fpe) and Urban (hi 20s fpe) do well with the BBT 30g at lower velocities low 630-675+ fps... 1 :19 twist...Have not tested K.O and H&N in the Urban... The Buc is very accurate with the K.O. and the 30g H&N are tied with BBT for second place... it does .6" and below with the 27g H&N... did not like the lighter H&N...
lastly my MK 1322 pumper tosses the 30g BBT out of a Disco 1:16 barrel at about 550 fps does .6-.75 at 30 yards... got quite a few 2 moa lobbing them at 50 yards with a lot of holdover...
Way back when the BBT 30g were just in design I think I was the only one that was advocating that it would be stable in a 20 twist barrel saying that I had to go with my gut on this one rather than the faster twist recommended by the Kolbe calculator... also felt the predicted B.C. was a bit low...
a lot of what I do is sub optimum but still useful...
but is what what I wanted to prove out when I asked Bob to draw up some light .25 and .22 BBT... I wanted to have the independence of casting my own ammo and show it can be done at stock and near stock fpe levels...and in choked barrels also...
wanted to prove that slugs were not only for higher fpe .257 and up... high air usage and high $ rifles...wanted to show it was accessible and even useful on a budget and could be done in the small caliber rifles... not just big bores...because that is how it was back then...
I think PakProtector was the only one working with .22-.224 back then... but he was concentrating up around the 100 fpe level...
quite a few folks have proved it now... and even lighter rounds have come out now thanks to Nick at NSA... And now H&N JSB and others have followed suit...What I wanted to see has come about... it is a very accessible endeavor now...
I have kept to near stock and my rifles still do very well with pellets...I value that... not hard to hand pump for...it is a Niche I like...
ok so long winded and somewhat rambling...
to the shorter of it
rather than asking about barrel length to guide twist rate choice... decide the platform and how hard you want to push it to get your fpe from it... cause there is a whole range of fpe a 22" barrel can get... but remember the higher the fpe the more air used and muzzle blast can be a factor... Fpe should be decided by what you are trying to accomplish...
Me I am happy that I got 1" groups at 30 yards in really nasty conditions recently...It was with my Storm .177 with JSB K.O. 13.43...Tho my Storm is pinpoint accurate with 10.3 AA could not call what they were doing groups in those conditions...
So even just being able to hunt to about 25-30 yards in bad conditions at 20-25 fpe with a .177 is of value...(even tho do not know what twist rate is)
so you want to use a 19 to 22 inch barrel and you are getting 35-40 fpe... and you want around 1000 fps well you are not going to get there easily with heavy slugs.. ;)
to me a heavy slug is 35g-40g and over... at 950-1000 fps that is 70-80+ fpe...not easy to do with a 22' barrel...
so lets call 25g-30 heavy slugs... well now you are talking about 56 fpe to get 1000 fps w/25g and 67 fpe for a 30g round to hit 1000 fps... may get there with a highly ported rifle and a 22' barrel... will use a lot of air and at 22" a lot is going to go out the barrel after the slug has left the barrel...so expect a nasty muzzle blast...
twist rate well 19-20 will work... but 16 may also... though it may be badly over-spun at 25g... at longer ranges with some slugs that may matter a lot... some slugs not so much...
so you ask a simple question... well the answer is not so simple in reality.. ;)
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here is some .22 work...
https://www.gatewaytoairguns.org/GTA/index.php?topic=161995.20 (https://www.gatewaytoairguns.org/GTA/index.php?topic=161995.20)
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Thanks, guys.
There's an awful lot to digest here. ( ! )
Personally I think I'll leave it up to the manufacturers to get all this right, but this is great info for choosing the right model and options, down the road. I know that FX has been experimenting a lot along these lines. Not sure about any other companies as of yet.
It's also not easy to get details like twist rate, magazine length, etc for most rifles, but maybe that will change as awareness increases over time.
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so been thinking a (dangerous thing) what are the trade offs to K.O.ing a 30g BBT decking the boat tail off... Why to lose some SD/weight...get the velocity up out of my Buccaneer... and it likes the JSB K.O. ...
currently not turned up all the way... 675 fps with 30g BBT... 725 25g w/K.O.
so sacrifice the Boat tail from the BBT... hollow-pointed should be down around 25g
so debate becomes... how effective is the short rebated boat tail..? Me I like the clean base it provides because the rear of the round steers... but also a short gas check or short rebated Boat tail can help divert the muzzle blast evenly...and that can help prevent yaw at the muzzle... that can be significant in shorter barrel... and the Bucc
so losing those things and gaining maybe 85 total fps leaning on the hammer spring... Hmmm.... 760 ish fps flat base(? B.C. .065 ish ::)) vs 700 ish fps 30g .08 ish B.C.
I do know BSA has always been 1:19 twist for all their barrels... it was in the meteors owners manual... it's in this vid... so have to respect that they do care to share...plus have wanted a BSA since a friends BSA/Martini tilt block 22
so will compare the two rounds as shot out of my Buc .22 first...yep Like my BSAs
then as out of My .22 Max and .22 AT44 L they about match fpe...
yep Like my BSAs
https://www.youtube.com/watch?v=jhm1HnJkDfQ (https://www.youtube.com/watch?v=jhm1HnJkDfQ)
so blue is 30g .08 BC at 700 fps and green is 25g .067 BC 760fps
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Ok now out of my Max And AT44...looks a bit more worth the effort... only becuese the 30g is just a touch to heavy for the Maximus... so it make 5 fpe more with the 25g jsb K.O.
50 fpe with the K.O.'d .25g (green)... 45 with 30g BBT Blue
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so about what would look like and a Cg and weight check
http://www.geoffrey-kolbe.com/cgi-bin/drag_working.cgi?unit_length=inches&weight_unit=grains&bullet_name=.338+air&re_calculate=yes&boundary_layer=L%2FT&entry=return&diameter=.217&length=.325&nose=.114&meplat=.152&drive_band=.217&base_diameter=.170&angle=0&boat_tail=0.074&secant_radius=&weight=147.2&density=11.4 (http://www.geoffrey-kolbe.com/cgi-bin/drag_working.cgi?unit_length=inches&weight_unit=grains&bullet_name=.338+air&re_calculate=yes&boundary_layer=L%2FT&entry=return&diameter=.217&length=.325&nose=.114&meplat=.152&drive_band=.217&base_diameter=.170&angle=0&boat_tail=0.074&secant_radius=&weight=147.2&density=11.4)
looks like ~h 24.6 just from decking... so ~23g as hollow point...
http://www.geoffrey-kolbe.com/cgi-bin/drag_working.cgi?unit_length=inches&weight_unit=grains&bullet_name=.338+air&re_calculate=yes&boundary_layer=L%2FT&entry=return&diameter=.217&length=.251&nose=.114&meplat=.152&drive_band=.217&base_diameter=.217&angle=0&boat_tail=0.&secant_radius=&weight=30.9&density=11.4 (http://www.geoffrey-kolbe.com/cgi-bin/drag_working.cgi?unit_length=inches&weight_unit=grains&bullet_name=.338+air&re_calculate=yes&boundary_layer=L%2FT&entry=return&diameter=.217&length=.251&nose=.114&meplat=.152&drive_band=.217&base_diameter=.217&angle=0&boat_tail=0.&secant_radius=&weight=30.9&density=11.4)
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so been thinking a (dangerous thing) what are the trade offs to K.O.ing a 30g BBT decking the boat tail off... Why to lose some SD/weight...get the velocity up out of my Buccaneer... and it likes the JSB K.O. ...
currently not turned up all the way... 675 fps with 30g BBT... 725 25g w/K.O.
so sacrifice the Boat tail from the BBT... hollow-pointed should be down around 25g
so debate becomes... how effective is the short rebated boat tail..? Me I like the clean base it provides because the rear of the round steers... but also a short gas check or short rebated Boat tail can help divert the muzzle blast evenly...and that can help prevent yaw at the muzzle... that can be significant in shorter barrel... and the Bucc
so losing those things and gaining maybe 85 total fps leaning on the hammer spring... Hmmm.... 760 ish fps flat base(? B.C. .065 ish ::)) vs 700 ish fps 30g .08 ish B.C.
I do know BSA has always been 1:19 twist for all their barrels... it was in the meteors owners manual... it's in this vid... so have to respect that they do care to share...plus have wanted a BSA since a friends BSA/Martini tilt block 22
so will compare the two rounds as shot out of my Buc .22 first...yep Like my BSAs
then as out of My .22 Max and .22 AT44 L they about match fpe...
yep Like my BSAs
so blue is 30g .08 BC at 700 fps and green is 25g .067 BC 760fps
I tried H&N slug testpacks in .217 and .218 from two .22 BSA barrels at 12 fpe and at 36 fpe and the accuracy was poor. That kind of makes me cross BSA barrels off the list of slug shooters.
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wonder if N.O.E. would be interested in making it... seems could just use the 30g BBT cherry just with less plunge depth...he did shorten the .172 round...
RKR Buccaneer slug test 40 yards at bottom link reply 17 KO and H&N .217 bout 30-32 fpe...
honestly I think the KO shoot more accurately out of my .22 Buc than the 30g BBT because of harmonics and not the velocity outside the barrel...
But could be the Nose shape of the K.O. is better suited to the choke... but then I look at the behavior of the H&N slugs at 40 yards... and need to figure out why they are behaving the way they do...
the 25g H&N slug does not do well at all... the JSB 25g slug does... why..?
the 21 g and 23 g are all over the place(over spun + harmonics +?)
the 27g and 30g H&N slugs do ok but hit way higher... BBT 30g and H&N 30g are shooting about the same... same general pattern in multiple test...
maybe I will make some measurements/drawing and think about it some more...
it seems hard to know right now... So still seems like a bit of voodoo at times...
would 24 twist of some of the .22 short barrels help the light slugs... seems that would be the sweet spot for a light slug/pellet barrel..?
https://www.rimfirecentral.com/threads/rate-of-twist-the-22-short.593288/ (https://www.rimfirecentral.com/threads/rate-of-twist-the-22-short.593288/)
in the end we have to test what we have available and compare notes...this is pretty new stuff still...and in the end your rifle/barrel will like what it likes...
I do think I will go for more fpe in the long run Probably with a 26"-28" barrel tho... more efficient and less muzzle blast than trying to do it with a 19"-22" in barrel...
https://www.gatewaytoairguns.org/GTA/index.php?topic=199308.msg156325640#msg156325640 (https://www.gatewaytoairguns.org/GTA/index.php?topic=199308.msg156325640#msg156325640)
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My.177 Buc really likes the .177 13.4g K.O. ....
30g BBT out of the Urban are not pin point but are near the same as the 30g BBT and 30g H&N from my 22 buc... just 50 fps slower... I consider that something that will get you dinner... so worth doing... best/optimum no...
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Riku sorry forgot to ask if you lubed them..?
right now I Am shooting only outside wind away from me only... am using a nano abrasive lube right now... trident divers grease which pretty sure is fumed silica and silicone oil...will clean and switch back to just 30 weight silicone after next session...
I imagine there is more lead in the blast than normal and safe but it does seem to have worked... no leading yet... only about 450 slug thru mostly BBT... about 80-90 with the silica the rest just 30 weight... still like the JSB 16 and 18g best and still same pellet friendly... a lot of cleaning 1/4 the time with goo gone... pull moderator crown protector in tight wet patch then 2 to 3 patches to dry...
have been lazy at times and just pushed a dry patch thru... cause sometimes it is just a few shots that day...but yep cleaned bout twenty times maybe even 30 for those 450 or so lubed slugs... it does not seem to matter to pellets accuracy wise if I clean after a lubed slug session(have mixed pellet/slug sessions)...but I at least drag one patch thru... I also clean out the moderator. saw no sense in putting out nano particulate when not needed...do not clean when sending pellets... just remembered I have some of the 18g Cudas forgot to try.. ;D think I will wait till next week.... It is a touch warm out... s' posed to be cooler then...
do plan on trying to move to a lard/beeswax type lube soon also... be cool if I can get home made more edible 30w to work... but am worried pellets might object...
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my Buc .177 ran ~30 abrasive K.O. in one sitting...cleaned well and since have sent bout 60 in a couple session just 20w silicone oil... no leading still loves the Sniper mags and CPUM and AA 10.3...
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Kirby,
Boy do I like the testing you are doing, as it tends to separate various factors that detract from accuracy that might otherwise be overlooked. It’s my belief that attention to cleaning, leading, and lubrication done properly is the final step towards ultimate accuracy. Hopefully you are able to work with the best bores available and nicely polished. I base this on all the work done with single shot black powder muzzle loading bench slug rifles from the early 1900’s with their fouling problems. I have done this in the distant past but am now to old to pursue it with pcp rifles.
One approach was the use of false muzzles when loading from the front, a 2”section of bbl was cut off and centered with four pins to the muzzle. The loading end was slightly enlarged to allow the gradual engraving of the projectile following the rifling when loading with a rod. The bore was always cleaned in exactly the same way between shots for consistency. In later days using cartridges cleaning and loading was done the same but from the breach end. But the projectile was always seated and engraved before inserting the powder filled cartridge case after the bore was cleaned. The best of these long range rifles often called ‘Creedmores’ on occasion would produce a 10 shot ‘string’ at what we now call moa to 1000yds.
Various lubricants made a difference. For me allox and bees wax 50/50% worked best including comparisons to more modern lubes of the time. Better lubes seem to be available now so you decide what works now.
What you seem to be doing closely follows what was done with early target bench rifles but now with modern pcp rifles. From what I read it seems many are content with the accuracy and velocity already achieved and you seem to be looking a step beyond. Cudos, and I wish you well..
Please continue.