High velocity accuracy

[Tomygun1]

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

[rsterne]

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

[Tomygun1]

Thanks Bob,
It sounds like we both are thinking that the ballistic coefficient is the firewall for pellets.   

[Tomygun1]

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.

[subscriber]

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:




The fix:

[ballisticboy]

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.

[Ribbonstone]

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.

[subscriber]

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.

[ballisticboy]

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.

[subscriber]

Thanks, Miles

[Ribbonstone]

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).

[Scotchmo]

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

[Doug Wall]

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.

[TroyHammer]

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.

[ballisticboy]

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.

[splitbeing]

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?

[ranchibi]

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!

[subscriber]

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.

[Tomygun1]

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.

[K.O.]

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...