GTA
All Springer/NP/PCP Air Gun Discussion General => Air Gun Gate => Topic started by: Motorhead on January 14, 2012, 05:52:22 PM
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Will try and keep this short ...
Based upon my experimenting with many round nose style .22 pellets it became apparent that at least in JSB's, weights above 15 grain LENGTH was increased. As were the H&N Barracuda pellets.
I know greater weight comes from more lead, but is there something to the accuracy equation due too longer/shorter pellet lengths ?
* Powder burners bullet length and weight go hand in hand with rifling twist rates ... So ??? ??? ???
Comments welcome,
Scott
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I have had guns that seemed to "like" longer pellets before.
As far as more weight coming with more lead, correct. But weight increase also directly affects length, since you really cant go anywhere else with the mass in a .22 . Gotta get longer to add more weight.
Seems like I remember a faster twist rate on slower, target rifles.
Seems like a longer pellet could increase drag somewhat, and where drag is increased, there is often a little more control....
God bless,
Farmer
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I would suggest doing an experiment. Shoot some skenco pellets compared to regular sized pellets. Those skencos are the longest ones I've seen.
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It all boils down to which pellet your barrel likes? ;D Usually the heavier, slower pellets are the more accurate pellets. ;)
Lee 8)
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Someone(can't remember who) figured out that 1:14" rate of twist is ideal for airguns no matter what caliber, pellet weight or barrel length.
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I guess it comes down to the ballistic coefficient and speed. A longer, heavier pellet will take a bit more twist to stabilize it, although I'm not sure that the extra length and weight between standard and heavy .22 pellets is enough to call for a faster twist. As to speed, a heavier pellet will remain stationary in the bore longer. This not only increases "dwell time" or how long it takes the pellet to leave the barrel, but it can upset a springer's shot cycle. Many are optimum with a certain weight and fit of pellet since the pellet gives the piston some resistance when compressing the air. A too light pellet will cause the stroke to go too far because it will leave the breech sooner, letting the pressure in the compression tube fall too quickly. Conversely, a too heavy pellet will make the piston stop sooner and you might not get full combustion of the "fuel" because the temperature of the compressed gas will not be as high. For the heavier pellet, the resultant loss of power might slow it down so that, at a distance where a normal weight pellet might still be stable, the heavier pellet will begin to lose it. It really all depends upon what the gun is designed to shoot. For example, a Diana 48 or 350 will "like" the heavier pellets in its caliber while a Diana 34 or R9 might not. The only way to tell is through testing.
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Lots of good points made.
* Below based upon having ability to change velocity (.22 PCP Rifle)
What I find odd or at the lease quite interesting, is that VERY near equal accuracy can be had with the JSB lighter 13 & 15 grain as the heavier 18 and 21 grain pellets.
The 13 & 15 JSB's are shorter pellets at @ .265" OAL, where as the JSB 18 is @ 3.04" OAL and H&N 21 is @ .335" OAL
With the rifles power set how it is now, the 2 heavier pellets are 1 MOA or less within 50 yards ( Test distance ) and the 2 lighter pellets much much worse !
*If I drop the power of rifle the lighter pellets tighten right up and make very similar grouping.
So is this a case of slowing down pellets going to fast better equal speeds to the heavy pellet being shot hotter ? .... Or are we seeing stability issues brought on by difference in pellets OAL ?
While I don't have a chronograph to share actual speed results verses accuracy :P what I am wanting to do is share my findings and discuss the why ?
A case of WHY is why CAN'T a HUNTING air rifle shoot the the lighter and FASTER pellets giving flatter trajectory and must be weighted down, slowed down with far more arch of shot too shoot well ???
Has to be in flight pellet stability ... what gives and WHY ?
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... is that VERY near equal accuracy can be had with the JSB lighter 13 & 15 grain as the heavier 18 and 21 grain pellets...If I drop the power of rifle the lighter pellets tighten right up and make very similar grouping.
Maybe I am misunderstanding your post but isn't this just the well-known stability/accuracy issue of pellets as they approach/break the sound barrier and may have nothing to do with size? That would explain why the light pellets do better with less power while the heavy pellets are fine with more power (i.e. they both are travelling slower than the destabilizing high velocity). Don't know what rifle/speeds you are using though and I may be misreading your post.
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... is that VERY near equal accuracy can be had with the JSB lighter 13 & 15 grain as the heavier 18 and 21 grain pellets...If I drop the power of rifle the lighter pellets tighten right up and make very similar grouping.
Maybe I am misunderstanding your post but isn't this just the well-known stability/accuracy issue of pellets as they approach/break the sound barrier and may have nothing to do with size? That would explain why the light pellets do better with less power while the heavy pellets are fine with more power (i.e. they both are travelling slower than the destabilizing high velocity). Don't know what rifle/speeds you are using though and I may be misreading your post.
No clue there speed, but certainly not CRACKING into or out from the speed of sound.
Question is WHY can't we stabilize and accurately shoot light & shorter OAL pellets FAST ?
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The pellet starts to wobble and become unstable before it passes the speed of sound.
If you look for accuracy, stay way below speed of sound.
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I said approaching/breaking sound barrier makes instability, but maybe that is not accurate.? Tom Gaylord says "Experience shows that diabolos are best if kept below 900 f.p.s." which is 200fps less than the sound barrier. Maybe the 900 fps mark is the threshold to keep it under.? Just throwing that out there and regurgitating Tom Gaylords observations.
edit: Benji, do you have a guess for the fps threshold for when they become innacurate?
I wonder if the cylindrical shaped pellets have a different threshold? Seems like it would be quite different?
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Might be over-stabilizing the lighter pellets? Driving them a bit too fast?
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A threshold?
Well, a lot of field target rifles hover around 800FPS so I guess that's ideal.
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To further expand on your experiment, you could try some Dynamic pellets. They are relatively long for their weight because they are made of tin (alloy?).
Nathan
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The transonic region starts at about Mach 0.8 (900 fps).... Once you exceed that velocity, it is possible to get locallized regions around the pellet where the airflow can be supersonic, even though the pellet itself is well below Mach 1.... Small shock waves can form which can disrupt the flight of the pellet and open up the groups.... at the extreme, this can cause the pellet to tumble, at least in theory, if the twist rate is too slow for the length of the pellet.... Here is a generic chart showing what happens to bullet stability around the speed of sound....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/Tumble.jpg)
This chart is for a bullet having optimum stability (1.5) at high velocities.... Right at the speed of sound the stability drops below 1.0 and the bullet would be expected to tumble.... At lower velocities it is once again stable....
In terms of stability based on length, longer pellets would, in theory, need a faster twist.... However (talking lead pellets here), as they tend to have a higher Sectional Density and Ballistics Coefficient.... they are less affected by wind.... so just from that point of view would tend to shoot tighter groups at long ranges.... I once heard that Lothar Walthar used a 450mm twist (1 in 17.7") for all their airgun barrels.... so I'm betting they have done their homework.... I'm guessing that twist rate is a non-issue in airguns for the most part....
Bob
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rsterne, very cool info.! So as I read the graph, a pellet at 1300+fps is much more stable than a pellet at say 900-fps. I didn't expect that.
It looks like 900-1200fps is the "unstable zone" where there is a lot of variability (even if only locally around the pellet). So from that graph, if you are going for power, you have to go for MUCHO power (1300+) or live with <900.
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if you are going for power, you have to go for MUCHO power (1300+) or live with <900.
Correct.... and the pellet has to STAY above ~1200 fps all the way to the target as well.... The problem is that the BC drops off so dramatically as you approach the speed of sound that staying supersonic is nearly impossible with a diablo pellet.... I measured the BC of an 18.1 gr. JSB Heavy at only 0.020 at 1000 fps (about half of it's peak value).... and based on the measurements I did for other JSB Exact series pellets, they were down to about 0.010 at 1100 fps.... I didn't test at 1200 fps or more, so have no data.... but I would really doubt if the BC suddenly goes back up significantly above Mach 1....
If you launch a pellet with a BC of 0.010 at 1500 fps, it is down to Mach 1 at only 23 yards.... :o
Bob
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That makes total sense. I have a Q about Ballistic Coefficients. Does the tail, or skirt, really affect BC or just the pellet's nose? I'm wondering if BC numbers really capture how a pellet is expected to behave or are they just a measure of how aerodynamic the nose is. For example, would a diabolo with a narrow "waist" have the same BC as a more cylindrical pellet (no "waist" or narrow "waist") if the nose, weight, length were identical?
What about if they were identical in every way except for the length and shape of the skirt?
I'm learning a lot from your posts! Thanks Much!
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The transonic region starts at about Mach 0.8 (900 fps).... Once you exceed that velocity, it is possible to get locallized regions around the pellet where the airflow can be supersonic, even though the pellet itself is well below Mach 1.... Small shock waves can form which can disrupt the flight of the pellet and open up the groups.... at the extreme, this can cause the pellet to tumble, at least in theory, if the twist rate is too slow for the length of the pellet.... Here is a generic chart showing what happens to bullet stability around the speed of sound....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/Tumble.jpg)
This chart is for a bullet having optimum stability (1.5) at high velocities.... Right at the speed of sound the stability drops below 1.0 and the bullet would be expected to tumble.... At lower velocities it is once again stable....
In terms of stability based on length, longer pellets would, in theory, need a faster twist.... However (talking lead pellets here), as they tend to have a higher Sectional Density and Ballistics Coefficient.... they are less affected by wind.... so just from that point of view would tend to shoot tighter groups at long ranges.... I once heard that Lothar Walthar used a 450mm twist (1 in 17.7") for all their airgun barrels.... so I'm betting they have done their homework.... I'm guessing that twist rate is a non-issue in airguns for the most part....
Bob
This is EXACTLY what I was wanting to know with the inquire about power/speed . The speed boundary that exists above and below the speed of sound SADLY is where air rifles operate :P So .... even if you launch well above the speed of sound (200-500 fps) as is about it in air rifles the pellet slows down so quickly within the usable range it re-enters this instability boundary of subsonic flight.
But the LENGTH issue question still stands ? ... Is it as simple as twist rate matching to projectile length and FPS same as powder burners ? ..... Sounding like it is having barrel twist rate established for the AVERAGE pellet weight and length most will be shooting :-\
THANK YOU !! .... Great stuff there ;)
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IMO, the shape of the nose is the predominant factor in the BC.... assuming the SD (and therefore the weight) is the same.... IF there is a noticeable difference in the BC between guns shooting at the same velocity (and that may or may not be the case).... then that difference must be due to the skirt deformation, wouldn't it?.... I can't imagine the difference in the rifling grooves engraved in the head of the pellet making much difference from gun to gun.... but I've been wrong before.... :-[
The testing I did on the JSB Exact series pellets at different velocities was quite an eye-opener.... In case you haven't seen it before, here is a graph of the trends....
(http://i378.photobucket.com/albums/oo221/rsterne/Important/JSBBCs.jpg)
Those curves were hand drawn through only a few data points for each pellet, so don't use them to determine an exact BC.... but the trends are pretty clear.... I didn't have a powerful enough gun to see the "roll-over" point with the 25.2 gr. Monster but I would guess around 900ish?.... Incidently, I tested an RWS Hobby at 1131 fps (nice sonic "crack").... at just 5 yards, it had lost 96 fps and the BC was only 0.007.... It lost over 16% of it's energy in just that 5 yards.... :o
Bob
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Great info,Bob.
Thanks.