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mod90 · Veteran Member

I've been thinking a lot about the effects of velocity, particularly drag, in regards to projectiles fired from air guns(thinking rabbit magnum ammo, not necessarily diabolo pellets) and the issue of instability in trans sonic flight and it got me to wondering. How high would the velocity need to be for a 14-18 grain projectile of .177 caliber to lessen the effects of trans sonic drag and remain stable and accurate out to longer distances?
If a velocity of 1300-1400 fps could be attained with such a projectile to be sure it is far enough above the speed of sound to have an opportunity to go some distance before falling into the trans sonic region , and with a fast twist rate in a long barrel (I'm thinking somewhere between 1:9-1:12) would this lessen the negative effects of trans sonic drag to the point where such a gun could be usable for hunting or long distance target applications.
Now I'm aware that for such a projectile to be able to be pushed to such high velocities the air volumes and pressures that would be needed to reach such speeds would make such a gun a real air hog and the drop in pressure would quickly manifest in reduced velocities, but this could be compensated for with a larger volume reservoir at a higher pressure with a straighter or more streamlined air transfer process.
Any ideas or thoughts are welcome. I really hope someone can offer some insight.

rsterne · Moderator

I measured the BC of a .22 cal Rabbit Magnum at 750 fps to be 0.072.... It was so wildly innacurate at 25 yards it took many shots to put 5 across the sensors of the Chrony, and several hit the wire supports as the groups were about 6" across at that range.... The bullets did not appear to be keyholing, however.... That BC is high enough, providing it was still valid at 1300 fps (A HUGE IF) to put the bullet supersonic out to about 75 yards, ie similar to a .22LR high velocity.... You would have to measure the BC at that velocity to determine if that was the case or not....

To determine the twist rate for stability, I suggest you use the Border Barrels calculator at http://www.border-barrels.com/barrel_twist.htm

This is the only twist calculator I have found that addresses the transonic region.... Here is a generic graph showing what happens to bullet stability in that velocity range....

In this example, for a bullet having normal stability when supersonic (ie stability factor of 1.5) the stability drops below 1.0 (unstable) as the bullet drops below the speed of sound, even though it would be stable at velocities below 1000 fps.... I'll dig out the .177 and .22 cal Rabbit Magnum pellets and run them through the calculator and post the results....

You are correct that the biggest problem would be air usage in generating those velocities.... There is a theoretical limit of ~1650 fps using compressed air, and although it has been exceeded (by a .177 Condor shooting a 1.7 gr. plastic sabot at 1705 fps) the efficiency was only about 3%.... The 1650 "limit" is based on the RMS velocity of air molecules at room temperature, but the velocity distribution is a curve, and a small percentage of the molecules are moving fast enough to still provide a "push" to accelerate a pellet slightly past 1650 (at least that's my theory).... In practical terms, when you start playing with a PCP, you will find the amount of air required increases exponentially past the speed of sound.... Of course you could always use Helium (the atoms have a higher RMS velocity)....

Bob

rsterne · Moderator

.177 Rabbit Magnum: Required minimum twist 1 in 14"....

.22 Rabbit Magnum: Required minimum twist 1 in 20"....

Those twist rates are to provide a stability of 1.5 (recommended minimum) throughout the entire range of velocities, including transonic.... Click on the images to enlarge....

You might also consider the following thread.... http://airgunhome.com/agforum/viewtopic.php?t=8636 .... SeanMP and I are currently working on rifles to shoot the RWS 41.5 gr. .224 cal bullets using a 1 in 14" twist .22 Hornet barrel.... Our target velocity is 950-1050 fps (83-102 FPE).... therefore equalling the performance of a .22LR subsonic (target) load....

Bob

mod90 · Veteran Member

So essentially from what I can see a 10" twist would be sufficient to stabilize such pellet in flight. It would drop into the transonic range pretty quickly, but it would most likely reach a target at 100 yds before this effect could manifest. This is what I got using .177 cal at .270 long.

Only thing left to figure out is how much drag would the added friction of a 18" barrel create on the projectile, since it would be making more than 1 full rotation while still in the tube, and if the last 8" were left as smooth bore if it would lessen this effect.[/img]

rsterne · Moderator

A 10" twist is MUCH too fast for a .177 cal bullet that is 0.270" long.... Stability numbers of over 4 are considered "overstabilized" and emphasize any inaccuracies in the bullet causing deviation in flightpath due to increased precession.... Your graph shows stability of 4.0 at transonic and over 7.5 during supersonic flight.... WAY overstabilized....

The .177 Rabbit Magnum is 0.325" long and it only requires a 14" twist at transonic (for stability >1.5).... For supersonic flight, the stability factor is over 2.5 which is PLENTY.... Shorter bullets need a slower twist rate....

Bob

mod90 · Veteran Member

So essentially I'll be looking for a significantly slower twist with a slightly longer bullet. Very interesting. So then the shortest twist would be about 12-13 inches with a slightly longer bullet(.331") But what about the weight factor? Wouldn't the slightly longer, and thus heavier, bullet need a slightly faster spin rate? Or should its weight be enough to compensate for the inherent drag and turbulence it will encounter?

rsterne · Moderator