Range Report Twist Rates

[Blaine Fields]

It's been a while since I've posted, but the last few times I saw the issue of barrel twist rate come up there never seems to be included a discussion about atmospheric conditions as effecting bullet stability. But the fact is that whether a given bullet is stable upon exiting the barrel depends not only upon its rate of spin but the density of the atmosphere it is entering. So, it is possible for a rifle to have stable bullets on warm days at, say, 3000 feet but have the same bullet go unstable at sea level on a cold day.

The problem is that figuring out the stability factor for a given bullet is not a trivial task and most of us do not have the equipment or knowledge to obtain the types of bullet measurements we need to calculate the stability factor for a given barrel twist and the associated atmospheric conditions. (For the types of bullets we use in long range shooting, the Greenhill formula is completely unless.) You can see the Gyroscopic Stability Factor equation here to see what I'm talking about: http://www.nennstiel-ruprecht.de/bullfly/gyrocond.htm

A bright fellow by the name of Don Miller has, over the last couple of years, published in Precision Shooting magazine an approach to simplifying the stability equations. He has made some assumptions and simplifications in an effort to get rid of some of the more difficult measurements and has come up with some very usable and useful formulas that yield the stability of a given bullet for a given twist rate (or vice-versa) in defined atmospheric conditions. The results are close to the more rigorous approach. Actually, very close. But they are not identical. So, use these formulas with a little caution and good sense.

I've packaged the formulas in a program you can download. The download comes with two programs: one (the .EXE file) is for the desktop machine, the other (the CAB file) is for a PDA running WM 6.0. It may run on earlier versions but I haven't tried so don't know for sure.

The desktop program does not install so you can run it from your Desktop or where ever you decide to put it - at least that is the way it works on an XP machine; it doesn't read from or write to any files so there shouldn't be an issue with security. (I say "shouldn't" but who knows what a Vista machine will do?) The PDA program does install and does have read/write capability so you can save your work and open the file later. Go to http://www.lextalus.com/support.html and click on the TwistCalc link to download the zip file.

And remember, if you are going to specify a twist rate to just get the bullet stable at 1.1 stability factor, the formulas are "approximate" and you could be buying a barrel that will make a fine, but expensive, tomato stake. So, give yourself some cushion in these calculations. (Your stability factor MUST be 1.0 or more. A bullet that is not stable at the muzzle will never become stable.) When you run the program for some known military rounds, you'll see that the military generally wants a stability factor of about 1.5 at Metro conditions. The question for me was, what happens under very cold sea level conditions and at 15,000 feet on a hot day? We have troops these days who engage in combat at 15,000 feet and I was interested in seeing what the stability factor of the bullets was under those conditions.

Why not just specify a stability factor of 2.0 and not worry about it any longer? Well, you can but as the stability factor increases, so does the rate of spin and the angle of repose which means greater rates of spin drift. (Example: A .308 Win shooting a 175 Sierra MK in a 1:12 barrel will experience about 8" spin drift in 1000 yards; the same bullet will drift 12" (a 50% increase) in a 1:10 barrel.) Anyway, you can make those judgments once you know the stability factor for your rifle, bullet and shooting conditions.

 

[Graham]

Re: Twist Rates

Thanks for that.

If memory serves, didn't some benchresters get surprised in the mid 90's when they took their very slow twist barrels to a championship in colder climes? Since then I have always preferred just a bit extra twist than I need for the neaviest bullet that I want to shoot.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Blaine Fields</div><div class="ubbcode-body">A bullet that is not stable at the muzzle will never become stable.</div></div>Is there such a thing, then, as 'over-spun': My latest 10-twist .308 had a bit of trouble, not at 1000 but at 100: stability problems cropped up with 170gr Lapua factory ammo not grouping well and entering the target from right to left. The same ammo settled down after 300 and shot .5 MOA at 600.

 

[MitchAlsup]

Re: Twist Rates

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Blaine Fields</div><div class="ubbcode-body">And remember, if you are going to specify a twist rate to just get the bullet stable at 1.1 stability factor, the formulas are "approximate" and you could be buying a barrel that will make a fine, but expensive, tomato stake. So, give yourself some cushion in these calculations. (Your stability factor MUST be 1.0 or more. A bullet that is not stable at the muzzle will never become stable.) When you run the program for some known military rounds, you'll see that the military generally wants a stability factor of about 1.5 at Metro conditions. The question for me was, what happens under very cold sea level conditions and at 15,000 feet on a hot day?
</div></div>

The military specifies a twist rate such that a troop can use his rifle with issued ammo at -50dF under high pressure in a white-out. The point-5 in the 1.5 gyroscopic stability is to deal with the higher veicosity of the atmosphere in cold conditions at higher than average pressures.

Benchrest shooters (where 0.05" of group size gain is "worth it") use barrels that are just barely overstabilized (clost to 1.15). It is true that occasionally they will encounter conditions where their bullets are not fully stabilized--and on those days that gun will suffer.

Conversely, overstabilizing a bullet only has consequences far down range as the nose of the bullet rides high (right hand twist) and adds to spin drift to the ballistics.

I disagree with the statement where a bullet that is not stable at the muzzle will never become stable. Consider a bullet that exits the muzzle with a SF=0.99. Technically, it is unstable, but by the time the overturning towque on the nose makes the bullet tumble, the decelerating force on the bullet as a whole decelerates the bullet into a velocity range whare that spiin does stabilize the bullet. That bullet might take some time to settle down and print good groups, but it does become stable. On the otherhand, a bullet launched with an SF=0.9 cannot decelerate fast enough to ever become stable.