Ballistics question (air rifle)

Factual Questions
1

(I think there may be a factual answer, but mods feel free to move it to IMHO, if this becomes complete conjecture).

I purchased Benjamin Marauder air rifle in .22. Pre-charged pneumatic airguns are very new to me, as in never had one before. I’ve been shooting conventional guns since I was 9.

While this new gun is very accurate, I notice something odd. The gun shoots very well at pressures between 2-3000 psi. The groups are very good IMHO. Dime size groups at 100 feet.

As the pressure in the gun gets lower, I expect the velocity to drop of course, and POI to drop as well. What is odd, is that when the pressure and velocity gets lower, POI not only drops, it pulls left. Still a nice tight group though. It drops about 1/2 inch and goes left about 1/2 inch.

It has a 16x scope, and I use a sandbag rest. Still great groups, but why is it pulling left at lower velocities?

What’s up with that?

2

I am not an expert on guns or ballistics, so take this for what it is worth.

My suspicion is that air rifle “bullet” is light weight and is much more susceptible to wind than normal bullets. The left movement you are seeing is probably caused by wind. Have you tried shooting in the opposite direction and see if the bullet turns right ? That will confirm the wind theory.

3

Yes, makes sense. But it was a calm day. Might have had a little breeze. That might do it. I need more data I suppose.

4

Thinking more about this, the little bit of breeze did not change during the shooting of 50 pellets. Five targets not but 3" inches away from each other side to side. 10 shots per target. They got consistently worse pulling left.

5

I believe it’s due to Gyroscopic (spin) Drift.

When a bullet spins CW (as viewed from the gun), the bullet drifts to the right. The higher the velocity, the faster the bullet spins, and the more it drifts right.

In your case, the velocity is decreased, the bullet spins more slowly, and thus drifts to the left.

At least I believe this is what’s going on.

6

I thought that might be it and could not think of any other explanation. Kinda wonder why though? What’s going on here that spin (and the speed of the spin) causes it to drift in one direction or the other?

Thanks for the replies all.

7

I doubt you’ve a chronograph, but if you do, how much is the velocity varying from one pressure setting to another?

Besides Crafter_Man’s gyroscopic drift idea, could barrel whip be an explanation? Barrels vibrate from the stress of having a projectile go through them, and the muzzle moves around accordingly. Muzzle movements are consistent in accurate rifles for projectiles with similar velocities.

Perhaps the lower chamber pressure setting increases projectile barrel dwell time to the point that the projectile is now exiting the barrel when the muzzle is pointed down and to the left, from where it points when the chamber pressure is higher?

Edit: Article from Bryan Litz, who’s kind of a big deal in the ballistics world, on gyroscopic drift and Coriolis effect as it applies to shooting.

8

Nope no chronograph. The specs say at 3000psi, it’s shooting at 1000fps. Of course pellet weight and such would have to be taken into consideration.

It is kind of interesting, while I am using a sandbag for a rest, the barrel kind of floats on the gun. It is attached at the receiver, and has no other support. It does pass through a ring near the end of the stock, but the barrel is just a little bit smaller in diameter than the ring. If I push on the barrel at it’s end, I can move it about 1/16th of an inch side to side while the stock remains still. I have no idea why it’s designed this way, but looks to be deliberate. Strange IMHO. I rest the stock on the sandbag, not the barrel.

I sort of doubt an 18 grain pellet would have much effect on pushing the barrel around, but maybe so.

Thanks for the link. Very interesting. I need to find out which way my barrel twists though.

9

Quote from your link (First paragraph) : “ Gravity drag and wind are the major forces acting on a bullet in flight, but they’re not the only forces.”

Based on the above, shouldn’t we try to eliminate wind as a causal factor before going to non-major forces ?

10

What kind of pellets are you using, enipla? Here’s a list of ballistic coefficients the people at HardAir Magazine put together for typical air rifle pellets. Since for speeds 800-1100 fps the G1 ballistics model is close enough to the GA model HardAir used, I can plug the HardAir numbers into a free ballistics calculator like the one at JBM Ballistics, and see what wind etc are likely to do to the trajectory.

I’m going to go down memory lane with my old airgun experience, and assume you used something like H&N flat-nose pellets. They make a “Sport” model in .22. The stats for it, per HA Mag, are: Weight, 13.73 (grains, though it’s not explicitly listed in the chart); Diameter, 5.50 MM (which makes my teeth itch. Pick one unit system or the other.); BC (GA) 0.011 @911 FPS.

Let’s plug that in. I know you’re at altitude, but I forget how high. Is 5,000 feet close enough? STP otherwise. 10 MPH wind, full value. Zeroed for 10 yds, with a 1.5 inch scope height and I ran it out to 60 yds, using 3 yd increments. 1000 FPS muzzle velocity.

Doing all that, I get, at 33 yds (~100 ft.), a drop of +1.7 inches, and a windage of 4.1 inches. About half an inch per MPH of full value wind. Which is a LOT larger than it would be in centerfire rifle land. Do you have a Kestrel or other anemometer?

Making the MV 800 FPS instead, keeping everything else the same, and I know the BC changes with velocity, bear with me, and we get: +0.9 drop and 3.9 inches of windage. So leaving everything else the same, the group should shift a quarter of an inch down wind, and 0.8 inch lower. Assuming a 10 MPH constant wind.

I can’t tell the difference between 2-3 MPH just by feel, and at that range, it’ll make a difference. Thanks, am77494, ignorance fought.

For the Litz article, he mentions that gyroscopic drift is about 8-9 inches at 1000 yds. I doubt it’s what’s causing your drift, but I don’t know. Coriolis is about 2-3 inches at 1000 yds. They stack in the N. Hemisphere for most rifling twists. (RH)

EDIT: I’d always thought that free-floating barrels were the way to go for best accuracy? So what you’re describing enipla, sounds like a desired configuration. I was always taught to rest the rifle fore end on the front bag, not the barrel. So that sounds good too.

11

I’m using JSB Diablo Exact Jumbo. They get good reviews from what I see.

I don’t think wind is the issue here. First group of 10 spot on. Next group just as good. Next group down and left just a little, Next group down and left a little more. Next group down and left more. And so on. After Five groups of 10 shot within 15 minutes of each other, the pattern is clear. As the pressure goes down, it continues to pull further left.

I’m at 11,200 feet, so a bit higher altitude. ;).

Thanks for the link to the ballistics calculator. I’ll try to do some more shooting this weekend. I’ve been wanting to push this out to 200 feet just for fun.