7.62x39 Twist


PDA






mountain_cowboy
July 28, 2006, 02:00 PM
What would be the optimal twist rate for commercial ammo in a bolt action 7.62x39? I'd just like to know what would be best in terms of all-around accuracy with a wide variety of bullets.

If you enjoyed reading about "7.62x39 Twist" here in TheHighRoad.org archive, you'll LOVE our community. Come join TheHighRoad.org today for the full version!
Father Knows Best
July 28, 2006, 02:20 PM
The basic Greenhill formula for velocities over 2,000 fps is Twist = 180 x D2/L, where D is the bullet diameter (in inches) and L is the bullet length (inches).

A 7.62x39 bullet is .311 diameter if I remember correctly. You would need to measure your selected bullet to determine length, as I couldn't find that info online from any of the major bullet manufacturers. Typical 7.62x39 bullet weights range from 123 to 150 grains, and you may be able to build loads using even larger bullets.

Different manufacturers use different twist rates for their 7.62x39 rifles. In general, you can't go wrong with a faster twist. I have seen some as slow as 1:12. I think Ruger uses 1:10. Personally, I'd think that 1:8 or 1:7.5 would work well and permit use of heavier bullets.

Father Knows Best
July 28, 2006, 02:30 PM
I should add that I think the SKS has a twist of 1:240mm, which works out to 1:9.5 when converted to inches. I wouldn't go any slower.

Stiletto Null
July 28, 2006, 02:34 PM
1:9" works for ARs, it can work for you? :p

(Odd coincidence.)

Jim Watson
July 28, 2006, 02:36 PM
I think the usual iron curtain military rifle AK - SKS twist is about 10 inches, really four turns per metre. Not because the 123 grain bullet needs it but because that was what they were set up for after years of making MN barrels.

Are you going to have a rifle custom barrelled?
Otherwise you don't have much say on rifling twist.
If you are going to shoot commercial ammunition, I would not worry about it, largely because I don't think they make the ammo good enough for fine tuning to matter.

rockstar.esq
July 28, 2006, 09:46 PM
So you've reminded me of an article I read a considerable time ago wherein the author commented on the relatively lacking accuracy of the 7.62x39. Specifically he posited that the twist rate typically used for it was actually the same as that for the 7.62x54R which uses considerably longer bullets. Take it for what it's worth but I'd check out the twist rate of a 7.62x54R and attempt to avoid it!

Jim Watson
July 28, 2006, 10:44 PM
It is not the twist rate, nearly all .30-06 and a lot of .308s have 10" twists and they do OK. It is the quality. Are you getting match grade, commercial quality, or cold war military quality?

If you wanted to get the absolute best out of the case, I think a common rifle in benchrest score (not group) shooting is a .30 BR about the same capacity as .30 Commie Short, shooting 132 grain bullets out of a 12-14" twist. But that is not something you will find at the hardware store.

rbernie
July 28, 2006, 11:37 PM
I have loaded 180gr spitzer bullets @ 2000 fps in my 16" 7.62x39 AR with a 1:10" twist, and they're well stabilized to at least 200 yards.

rangerruck
July 29, 2006, 12:03 AM
I like the 1/9 myslef. But if you get an ak type, use foreign ammo. American made is 310 diameter, while european stuff is 311, usually groups much better.

jjohnson
July 29, 2006, 03:17 PM
Yeah.... I had a boltgun made up on a Mauser action for this. I bought a barrel in .308 x 39 (yes, you heard me, it's a .308, not a .311) from Numrich Arms. .308 bore with the Russian case. It works. If you hadload, it works great:neener:

This works okay for factory AK ammo, even though it's squeezing the bullet a little. The massive M98 Mauser action is dealing with any excess pressure if I'm creating any - and I'm watching my primers... Handloads are its regular diet though, and since it's a true .308 bore, I have LOTS more options than if it were .311. I load "real 30 calibier" stuff anyway.

Anyway, this barrel has a 1 in 10 twist. I've been loading surplus .308 bullets, both in 147 grain boat tail and 150 grain flat base (both spitzer, fmj) and both of them are stabilizing just fine. Since a 1 in 10 twist works fine with 168 grains, that's a winter loading project for me, to see how this thing might shoot with actual match bullets. So far, though, 100 yard 3 shot groups are about one inch. Almost all groups are coming in the size of a quarter or less, so I'm satisifed. I haven't tweaked the loads much, but judging from the holes on the paper, they're for sure not keyholing or anything. Good luck.

If you enjoyed reading about "7.62x39 Twist" here in TheHighRoad.org archive, you'll LOVE our community. Come join TheHighRoad.org today for the full version!