Home Forums Hunting & Shooting Ask The Gunwriters Twist Rates

What effect does elevation have on twist rates. Does higher altitude require a faster twist to stabilize long bullets. Asking because just stared loading for a 25 06 with 1:10 twist Ruger #1. I've recall reading that the 115 grain Berger and 110 Nosler BT are a little unstable with 1:10 twist. We usually hunt above 5500 feet elevation.

Higher elevation usually requires less twist for the thinner air.

Don't know about the 110 Nosler BT. Nosler makes 100 and 115-grain .257 Ballistic Tips, so suspect you may be thinking of the 110-grain AccuBond. According to the JBM twist-rate program, either will be quite stable at 5500' above sea level, even in pretty cold temperatures--which is the other major factor in bullet stability.

It's pretty easy to calculate twist-rates these days, due to accurate computer programs. A lot of people use the Berger program, but it doesn't include any consideration for plastic-tipped bullets, which will stabilize in slightly slower twist-rates. Today's twist-rate programs are based on the Miller Formula, developed by the late Don Miller--but he later discovered that plastic-tipped bullets don't require as much twist as his basic formula predicts, because they're so light. The JBM formula includes an input for plastic tips.

I crunched a bunch of numbers from both the Berger and JBM formulas, and as a rule-of-thumb for the rifling twists used for most big game cartridges, 5000 feet in elevation equals about an inch in twist-rate. In other words, a bullet that will stabilize at sea level in a 1-9 in twist will stabilize in a 1-10 twist at 5000 feet.

I've never seen it quantified that way before. Pretty cool. Thanks, John.

Thanks!

I found it interesting as well--but then I'm obviously kind of curious! (In more ways than one...)

That's interesting. I had no clue that elevation had an effect on bullet stability.

As for stabilizing the 115 grain Ballistic Tips, they should stabilize in a 1:10 twist barrel up to 117 grains. I don't even know if there's a 117 Ballistic Tip. I just know that in my .257 AI with a 1:10 Twist that it stabilizes all bullets up to 117 and a few 120 grains. That's what the barrel maker said anyway. My .250 Savage has a 1:9 twist and it stabilizes all bullets up to 120 grains. I've never tried anything heavier. That's what Krieger told me it should do. The one 120 I've tried in it was a Speer and it seemed to stabilize it ok.
But for what I do I don't need a 120 in either rifle. The 115 grains do everything I need.

Filaman,

How are you defining "stabilize?

And no, Nosler does not make a 117 Ballistic Tip, but they do make a 115.

Thanks John. You're are right, I was thinking of the 115 BT.

Never a bad idea to spin em fast...

Mule Deer, my understanding of stabilize is the twist rate causes the bullet to spin at a certain rate. This spins the bullet fast enough where it will fly true like a football. If too long a bullet and too slow a twist the bullet will wobble and won't fly true.
However, I don't know how far the bullet will fly straight. If it is borderline stability, as it loses velocity at some point it will become unstable, and begin to wobble resulting in loss of accuracy from that point onward. That's my understanding of it anyway.

BTW, I normally use 115 grain Combined Technologies Ballistic Tips. They shoot very good in my two .25s.

Filaman,

You have stability backward, but many people do. Spinning bullets do not become less stable as they slow down.Instead they become more stable--at least at the ranges most people hunt big game.

This because there are two opposing forces on spinning bullets--the spin, which gyroscopically stabilizes the bullet, and air pressure on the front of the bullet, which destabilizes them. The rate of spin has to be sufficient to overcome the frontal air pressure (and yes, these are exactly the same forces at work on a thrown football).

The rate of spin produced by the rifling and muzzle velocity drops very slowly as the bullet passes through the air. The bullet's velocity, on the other hand, drops comparatively rapidly, which reduces the air pressure on the front of the bullet, making it more stable.

The main reason many shooters think a bullet destabilizes as it loses velocity is wind-drift--which increases about twice the rate as range. For instance, if a given bullet drifts 2 inches at 200 yards in a 5 mph wind, it will drift about 8 inches at 400 yards. This effect occurs even in very mild breezes, so mild many shooters don't even bother trying to compensate for wind-drift, instead aiming directly at the target. This results in larger groups at longer ranges, which many shooters believe are the result of bullets losing stability. But they're not.

There is an exception as bullet velocity drops below the speed of sound, about 1125 fps at lower elevations, which creates some turbulence. That's why .22 rimfire match ammo typically has a muzzle velocity slightly below 1100 fps: They leave the muzzle at less than the speed of sound, so don't drop through the "transonic zone," and long-range centerfire target shooters try to keep their bullets supersonic. But the velocity of most modern varmint and big game bullets does not drop anywhere near the speed of sound at typical hunting ranges, so become more stable during their entire flight.

Great explanation MD, thanks.

Thank you sir. That makes sense when you explain it. But you're right, there's a lot more than me that have that wrong. I need to study your explanation to get it stuck in my 72 year old mind. I hope you don't mind, I copied your explanation and put it in a file in my computer. I'm not a ballistician by any means but I'm very interested in ballistics. Just bare with me, I'm learning and I ain't dead yet dude !

We used to think that they did better as they slowed down but now we are learning, through the ELR shooting, that a bullet that is apparently stable (using doppler RADAR to monitor) will to to hell at certain ranges unless it has enough twist. A bullet that seems to fly great at 1000 yards will often yaw and wobble at 2000 yards. A lot of the ELR guys have good to very fast twist rates to combat this. 20x caliber is a good starting point with the copper bullets for true long range stability.

I have killed deer with rifles sighted in at long range in 25-06 and 257 Roberts Ackley rimmed, both with 115 gr Nos Bal Tips. Both with 10" barrels I threaded and chambered. No problems.

Yep, higher elevation means less drop too. Had that argument with someone before. But I promise you a 75gr Amax shot at sea level drops a hell of a lot more than the same bullet at 6000 feet.

Cold can really screw some stuff up, concerning twist.

I have a 260 AI that shot 130 Normas great at 5400' when i lived in Prescott, at 1300' they group 3-4". I guess they were on the ragged edge for 5400' .

I'll probably never order another barrel with less than 1:8 twist unless it's for a 458 or 9.3.

Clark,

Like almost all .25-caliber bullets made today, the 115 Ballistic Tip was designed to stabilize under just about any conditions in a 1-10 twist, even from a 1-10 .250 Savage.

That makes perfect sense, pressure is directly related to area which is a 'squared" function.(pi r^2)

Likewise cold air is more dense than warm air.

So here is the question is does elevation help the winners or the losers more because of the square?
Does the flatness of high BC bullet improve more or less than a low (dirty) BC bullet?

Why don't you check that out with one of the excellent computer ballistic programs available free on the Internet these days?

Of course, to do that, you need good ballistic-coefficient data, and one thing most average hunters don't realize is that BC not only varies with elevation/temperature/humidity etc., but with velocity and bullet shape. What may theoretically apply to a certain bullet with the standard G1 BC (basically a flat-based spitzer) will not apply to a G7 profile high-BC boattail.

Which is why I've been recommending Bryan Litz's excellent book, BALLISTIC PERFORMANCE OF RIFLE BULLETS, for several years. Not only does Bryan explain all of this excellently in terms most of us can understand, but he also lists the BCs of a bunch of commericial bullets from .17 caliber on up, at varying velocities and conditions. The BC's were not just derived by calculations but partly by actual shooting at his facility in Michigan.

I have used Bryan's BC's with a bunch of bullets when shooting out to 1000 yards at the local range, and in every instance they have been just about dead-nuts. You can order the book from www.appliedballisticsllc.com