The AirGun Guild

General Category => Ask Bob => Topic started by: Steelhead on October 29, 2019, 06:26:39 AM

Title: Spin drift revisted
Post by: Steelhead on October 29, 2019, 06:26:39 AM

Yesterday I went out for a quick shooting session to alleviate some boredom and anxiety. Fires and subsequent evacuations/power outages in my area have made things difficult on a lot of levels (the Kincaide fire is about 20 miles from as the crow flies)

I had 2900 psi in my tank so I took some NUAH targets out to the 200 yard stand to see what I could do in the wind (blowing sustained about 15 mph with stronger gusts)  I shot a 5 shot group at 3.6" at 200 and I'm not sure if that's NUAH worthy or not. Maybe you could answer that before I post it on the GTA Long Range/NUAH thread and embarrass myself.

On another note, I noticed a distinct difference in windage between my Arsenal 110 gr. FNGC and my NOE 134 BBT. This has been a common theme when playing around with different bullets. It's not a concern, but I'm curious as the 'why'. The difference yesterday at 200 yards was about 9" to 11" further to the right with BBT's, and about 3" to 4" at 100 yards. Is this 'spin drift'? My first thought is that it may have to do with bullet length/barrel twist. It confuses me and if I knew the answer it will make me a better shooter.
Title: Re: Spin drift revisted
Post by: rsterne on October 29, 2019, 08:24:09 AM
Your 3.6" group at 200 yards would be considered 1.8 MOA in NUAH, so as long as it resides inside or touching the 4 MOA ring (8" at that distance) it should qualify as Sharpshooter....

I have never made detailed calculations on Spin Drift, but the JBM calculator should be able to give you the answer.... Those deflections sound like they are NOT from Spin Drift, just off the top of my head.... more like simple wind drift....

Title: Re: Spin drift revisted
Post by: Steelhead on October 29, 2019, 03:10:52 PM
Hmmm. I think I understand, I just didn't look at it that way. I'm 'assuming' that each bullet is behaving in the same way with relation to the wind. In reality I'm sure it's not. The wind was quartering away mostly right to left with some gusts.

I've run into this same scenario with other bullets too; my windage shifts drastically (3" at 100, 6" or more at 200) when I change from one to another. I'm assuming (there's that word again) that my only major adjustment 'should' be the elevation. I see this in windless conditions as well.

Interesting stuff getting big bore airguns to behave consistently at longer ranges. Once you hit a sweet spot you can start dropping some amazing groups at 300+ with relative ease. The challenge of ever-changing variables keeps it interesting. When I don't want to think too much about it I just pick up the regulated pellet shooters for instant gratification!  ;)
Title: Re: Spin drift revisted
Post by: rsterne on October 29, 2019, 05:33:34 PM
It could also be that if the bullets are shot at different velocities, the barrel is vibrating differently, and throwing the bullets to the side.... There is nothing written in stone that barrels only vibrate vertically.... A significant muzzle weight, by bending the barrel down at rest, will encourage vertical harmonic movement over horizontal....

Title: Re: Spin drift revisted
Post by: Alan on October 30, 2019, 06:08:07 AM
I don't have enough experience with airguns to stand pat on their spin drift. However...

The only time I have experienced spin drift, is at extended ranges (≥300 yards) while shooting high-velocity firearms (≥3,500 FPS), with bullet spin rates around 280,000 RPM), and wind velocities over 5 MPH. You can generally calculate wind drift which causes horizontal displacement. But when you see vertical displacement as well, you have to assume it is spin drift. Especially so, if the wind direction is (near) perpendicular to the path of the bullet.

As I said, I can't speak about low velocity pellets and/or slugs.
Title: Re: Spin drift revisted
Post by: rsterne on October 30, 2019, 08:28:49 AM
Alan, what you are describing, a vertical deflection in a crosswind, is not spin drift.... it is "Aerodynamic Jump".... There is a good explanation of it in this article in HAM....

The longer the bullet and the higher the RPM, the more the vertical deflection in a given amount of crosswind.... In addition, the direction of that deflection depends on if the barrel is right or left twist, and of course the wind direction.... For a bullet in a RH twist, the groups in a gusty sidewind will be roughly from 10 to 4 o'clock instead of just horizontal.... Interestingly, pellets are the opposite.... A wind from the right at 3 o'clock will cause a drag-stabilized pellet in a RH twist to drop instead of rise....

Spin drift is a sideways drift that occurs at long range and it NOT related to the wind.... It is small, and will take place when there is NO wind, and is therefore very hard to separate from wind drift.... It is to the left in a LH twist and to the right in a RH twist.... The longer the bullet and the higher the RPM, and the greater the distance to target, the greater the spin drift.... The amount varies with the square of the distance, and for a sniper at 1000 yards it might be 8-10".... At 500 yards, 2-2.5".... and at 100 yards 0.08-0.10".... What I don't know about spin drift is how it affects slugs or pellets at much lower (subsonic) velocities.... I do know that if you are shooting at a constant range, it is always the same.... so when you sight in your rifle, you will compensate for it (at that distance)....

Title: Re: Spin drift revisted
Post by: Alan on October 30, 2019, 09:27:30 AM
I'll agree with that!

When I was "into" long-range shooting (prairie dogs), the usual sight in distance was 400 to 500 yards. Obviously, you've compensated for spin drift, out to at least 800 yards; a distance where you might detect it if you were looking for it.

Most folks never have the opportunity to shoot at really long distances, say 1,500 yards or more. This is obviously far beyond any (normal) airgun's capability. The 338/378 AI I had built, shot 285 grain, LD bullets at almost 3,900 FPS out of a 31 inch barrel. I killed a prairie dog with that gun, at 1,710 yards. It took 7 shots! Part of the issue was varying winds (≥5 to 7 MPH). I say this, because some shots were a bit high, some a bit low, some to the left, and some to the right, even though the same scope setting was used. I think I was as surprised as the prairie dog was, that I could hit it at all! Based on this, I admire those snipers who're capable of hitting a man-sized target at 2,500 yards, and do it twice in a row!
Title: Re: Spin drift revisted
Post by: Steelhead on October 31, 2019, 07:24:35 AM

Thank you for further explaining that. I'm grasping some of the physics at work here and your last post was awesome. I've put a ton of shots through my Texan in the relatively controlled conditions of properly sized bullets, clean barrel, consistent rests, fully regulated tether, no wind, measured ranges, etc.  Changes in windgae when switching bullets had me a little confused. Elevation variance was obvious (weight/shape/speed) but I couldn't wrap my head around the windage. I knew it was SOMETHING because it was consistent, but I couldn't put my finger on the WHAT. Your theory on how much bullet length/barrel twist rate effects POI. I also never considered that a barrel harmonic could be anything other than vertical.

I'm going to revisit the 'De-Harmonizing Barrel Weight Thingies' that I made a few months ago. I had meant to implement them at a shoot with Steveoh but I had a 'mechanical failure' and never used it. I JB Weld-ed a pic rail to the bottom of my suppressor to suspend a PVC pipe with lead shot/oil in it. As soon as I put it on the pic rail fell off. I'd forgotten that I had rattle canned the suppressor flat black and the since the pic rail was stuck to just the paint it fell off instantly. I haven't played with it since but I'm going to pick it up again next outing and see what happens. I think the Texan can benefit from it as the barrel is supported quite a bit and thus would not experience severe 'droop' like a free-floating version might.