OT Barrel twist rate and bullet weight

[pharmseller]

So what's the rule - the faster the twist rate, the lighter the bullet? I.e. 1 in 7 will stabilize a smaller bullet better than 1 in 12? How does twist rate affect optimized bullet weight?

P

[TedH]

No, just the opposite. A faster twist is needed for longer bullets.

It's also possible to "overstabilize" light bullets. I was loading for a 223 one time that had a fairly quick twist, think it was 1-9". I loaded some old Hornady SX bullets to try. When I shot the first group, they were not even on paper. I set up at 25 yards to see where they were going and just got a speckle of odd holes in the target. They were spinning so fast that their light construction couldn't hold together under the extreme centrifugal force, and they were exploding as soon as they got out of the bore! They shot very well at the same velocity from a 1-12" twist rifle.

[Chuck 100 yd]

What Ted said +1

Get the calculator out and see how fast a bullet spins when fired.
IIRC a 40 gr. .22 bullet if fired from a 1:7 twist at a possible 4,000 fps in the Swift or .22 Varminter (.22-250) is turning over 370,000 RPM.

[AJMD429]

One other thing to remember is that it isn't really "twist rate" that is important, it is RPM - that is determined by twist rate AND by velocity - a bullet going 2,000 fps through a barrel with 'only' 1:24 twist would actually be spinning faster (60,000 RPM) than a bullet going 1,500 fps through a barrel with a 1:20 twist (54,000 RPM). The problem is that unless they are lighter weight (i.e. copper solids) or unusual construction, the longer bullets which need the higher RPM will generally be the ones most difficult to drive faster. However, comparing the same twist rate in say a .357 Mag. rifle and a .357 Max. rifle, you would need to take the different possible velocities into account somewhat.

I'll be interested to see with the stupid "lead bans" and more use of copper bullets which tend to be longer ('harder to stabilize') but lighter weight for a given length ('easier to make go fast') how it works out.

Maybe we should all start shooting saboted aluminum bullets....

[pharmseller]

Thanks for the replies. The reason for the question is that a friend of mine wants to buy an AR-15 and he has different twist rate options. He asked me which twist rate would be optimal for the usual .223 bullet weights, so I'm askin' y'all.

P

[El Chivo]

I needed this topic - my next project is to try the Barnes XPB pistol bullet in the 35 Rem. They are 140 grain and I was wondering what effect the twist rate would have on the lighter bullet.

That's good info that it's really RPM that's important. For example, if I find they're wild at high velocity, I can slow them down a little, and see how they do.

[Pisgah]

Thanks for the replies. The reason for the question is that a friend of mine wants to buy an AR-15 and he has different twist rate options. He asked me which twist rate would be optimal for the usual .223 bullet weights, so I'm askin' y'all.

P

Arguably, the best compromise twist rate is 1 in 9, which does a good job stabilizing bullets from around 45 to about 70 grains.

[Buffboy]

Thanks for the replies. The reason for the question is that a friend of mine wants to buy an AR-15 and he has different twist rate options. He asked me which twist rate would be optimal for the usual .223 bullet weights, so I'm askin' y'all.

P

Arguably, the best compromise twist rate is 1 in 9, which does a good job stabilizing bullets from around 45 to about 70 grains.

I agree 1 in 9 is a good compromise but I got 1 in 7 with my latest AR. They are difficult to find right now and considerably more expensive than 1 in 9.

The why is: I don't shoot smaller than 55gr bullets in mine any more, they shoot just fine with that twist and you can shoot any bigger bullet with it. I've found that the cheap 55gr FMJ bullets kill remarkably well on coyotes and other larger predators if they are spinning really fast from the twist. I've never recovered one but they don't often exit either(I'm not in the habit of dissecting varmints) . I suspect they explode internally. That does not hold with the slower twists with FMJ, they just make a little hole in and almost always exit with that same little hole. Normal 55gr softpoints just evaporate even in prairie dogs with that twist. Unfortunately smaller weight bullets and "extreme expansion" 55gr bullets don't always make it to the target. They tend to break up in flight.

[AJMD429]

Arguably, the best compromise twist rate is 1 in 9, which does a good job stabilizing bullets from around 45 to about 70 grains.

Yep - you'll see some faster twists out there, but they are for folks who are shooting 80 and 90 grain .223 bullets that look more like pencils than bullets. (IMHO if you want to shoot a 90 grain bullet, do it in a .243 Win or 6mm Rem.)

[Gene Dip]

Here's a neat chart that Shilen recommends. They will discuss your needs also for specific bullets. I've dealt with them in the past for a 7mm Rem Mag. They're excellent people to work with.

http://www.shilen.com/calibersAndTwists.html

Good luck, Gene