Grain and twist rate

[Echo9]

I know that putting too heavy a bullet through a barrel with too slow a twist rate will cause the bullet to yaw mid-flight, like a 70gr .223 through a 1/12 barrel. But what is the effect of using a bullet too light for a fast twist rate?

Is it just a loss of accuracy? Or can the bullet literally spin itself apart, as I've heard? If that's the case, does it just spin the jacket off? Or does the entire bullet disintegrate?

I've seen .22LR conversions for ARs that don't include a barrel, just a magazine and a bolt. I'd imagine a lot of people with 1/9 or 1/7 barrels try it out at the range and have problems with 36gr ammo.

 

[dmazur]

I haven't heard of inaccuracy with light bullets. It takes a certain amount of twist to stabilize a bullet of a certain length. There are velocity "classes" as well, as I remember. As long as you're shooting lead, I don't think there is a correction for density.

As long as you have the minimum amount of twist, I think it works. More is OK, up until the point where the centrifugal force from the extreme rotational speed tears the bullet apart. Thinner jacket material (as in varmint bullets) is more of a problem.

However, I don't think the .22LR has enough velocity to do this, even in a fast twist barrel. I believe the velocities are around 3800-4000fps to get into this problem area.

(I don't have a link, so if I'm off on this it's because I have CRS.)

 

[MinnMooney]

The reverse is not such a 'hard-n-fast' rule. I shoot 40 gr bullets in my AR-15 with a 1:8" twist and it's highly accurate. The only limiting factor with a real fast twist is that you have to use bullets designed with a jacket that is heavy enough (read 'thick enough') to stand up to the extreme RPM/centrifugal force that is forced on the bullet. If you only load to 2800fps - 3400 fps......... no problem but at 3800 or 4100 fps like a .204 Ruger, well, now your looking at disintegration-in-midair problems. That's why most of the super fast calibers are given 1:12" or 1:14" twist rates.

 

[Echo9]

The reverse is not such a 'hard-n-fast' rule. I shoot 40 gr bullets in my AR-15 with a 1:8" twist and it's highly accurate. The only limiting factor with a real fast twist is that you have to use bullets designed with a jacket that is heavy enough (read 'thick enough') to stand up to the extreme RPM/centrifugal force that is forced on the bullet. If you only load to 2800fps - 3400 fps......... no problem but at 3800 or 4100 fps like a .204 Ruger, well, now your looking at disintegration-in-midair problems. That's why most of the super fast calibers are given 1:12" or 1:14" twist rates.

Super fast calibers? Or super fast light calibers, like a .220 Swift?

 

[MinnMooney]

Or super fast light calibers, like a .220 Swift?

Generally, it is the light calibers including 17 Rem, 204 Ruger, .220 Swift and .22-250 Rem that have the slower twist rates like 12" or even 14". Savage refused to produce a .22-250 barrel in anything faster than 1:12" until 2 years ago. Now they make a couple rifles with optional barrels that have 1:9" twist rates........... but you certainly can't buy any Savage .22-250 with 1:9".

The larger the caliber, the harder it is to achieve 3900 - 4100 fps apeeds.

 

[bgeddes]

The .223 is a goofy round, notorious for tumbling. The reasons why are argued, but typically it is a mismatch as you describe, IMHO. The shape and balance of the bullet may have more to do with it than anything.

 

[Red Cent]

http://kwk.us/twist.html