Twist Rates!

R

RemRexAnotherDay

New member
Hello

I've often wondered how much the twist rate a barrel has effects the impact or overall performance of different grain bullets. For instance, how well will a 60 grain bullet perform out of a 1 in 14" twist? And how much difference is there between the 60 grain and say a 40 grain bullet? Could someone explain to me the bullet weights in relation to the twist in a barrel?

So, what would you guys think of a 60 grain Nosler Partition out of a 22-250 with a 24" barrel (1 in 14" twist rate)in terms of stopping power and pelt damage?

Take care


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Steve

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My guns name is Rex, cuz when it touches one it wrecks there freaking day!
 
In brief, because it's past my bedtime.

The twist rate needed to stabilize a bullet, depends on velocity and bullet length. A given length bullet needs at least a certain rpm to stabilize it. At a higher velocity less twist is needed to stabilize, because at the higher velocity, the rpm is higher.

Over stabilization, too fast a twist, will cause poor quality bullets to be less accurate. This is not normally a significant problem , unless the bullets are really poor, or you are looking for that last .010 inch bench rest group.

Something you don't hear mentioned much is the effect twist rate has on bullet terminal performance. The difference is dramatic. A bullet which will make a 1" exit wound from a 14" twist barrel, will probably only penetrate half way through the same animal, when shot from a 9" twist barrel.

That 60gr will probably stabilize from a 14" 22-250 with a hot load, but it won't expand much.
 
That's a fair answer for being so tired, Jack! In fact, it should be read twice so that we don't miss anything.

I'm not too keen on Nosler partitions for use on predators. From an accuracy and terminal performance standpoint. Minor consideration B: cost. Your thoughts?

Good hunting. LB
 
Jack Roberts: Nice explanation of the factors in determining rate of twist. I wonder though, it just doesn't sound right that a bullet shot at the same velocity through a 9 twist won't even exit the critter same as one shot through a 14 or 12 twist. What are your thoughts on that? Seems to me that rotational velocity has very little to do with linear velocity, and all other factors remaining the same, linear velocity along with bullet expansion should result in very similar penetration.

Is it that rotational velocity affects bullet expansion or something? I have heard of the thin jacketed varmit bullets vaporizing a few yards from the muzzle in a fast twist AR15

Man do I have a fast twist on my Sharps, a 1 in 16" twist!

Maybe?


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Regards,

Sharps Shooter.

'74 Sharps... when you care enough to use the very best.
 
The rotational velocity, or in easier to understand terms, the rpm, revolutions per minute, has a dramatic effect on how a bullet expands.

Think about rpm for a minute. Most auto engines will explode at 6000 to 8000 rpm, because of the hugh centrifugal force. Extremely well balanced aircraft turbine engines can turn up to 120,000 rpm. Even a medium speed bullet from a 14" twist barrel will be turning over 100,000 rpm. A high speed bullet from a 8" twist barrel can easily be turning over 250,000 rpm. The centrifugal force trying to make this bullet expand is immense. Once bullet expansion starts, as by impact, the rpm will make a big
difference in how far and how fast it expands.

One good effect of rpm is that it is pretty much constant, regardless of range. It doesn't decay like velocity. It does decay some of course, but not significantly for even long ranges.

Bullet blowup, before reaching target, is a major problem with real fast twist barrels. Just the least roughness in the barrel is enough to cause blowups.

Actually the blowups take place about 30-50 yards from the muzzle usually. If you are behind the shooter and watching, you can usually see them as a faint grey cloud.
 
I agree with Jack but, I have a couple of points to add based on my personal experiences with the 22-250 AI and twist rates as they relate to bullet weights.

I recently rebarrelled my 22-250 AI to a 1-9 twist barrel, primarily because 90% of the time I shoot 60 grain bullets or larger, up to 75 grain. The finished barrel is 25 1/2 inches long.

Before I made the change, I talked to the folks at Shilen and based on their recommendations I chose the 1-9 twist. For the 75 grain and up they recommend a 1-8 twist. I also talked to the folks at Sierra and Hornady and got similar recommendations.

Due to their length, a 1-14 barrel will not stabilize them as well as the faster twist. Also, for most varmints I shoot the Hornady 60 grain VMax and it states right on the box to use a twist of 7-9. While the accuracy of the 1-14 barrel was acceptable with the larger weight bullets, the accuracy with the new 1-9 barrel is much better.

Just as an aside, the load I use drives the 60 grain VMax at 3625 fps, based on a 10 shot group thru an Ohler chronograph.

The performance of the 60 Vmax on the two coyotes I have taken since I switched barrels has been execellent.

One word of caution, the above is what works for me. It may or may not work for you.

Pecos
 
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