proper twist rate for 7.62x39 8.5inch barrel?

[Guest Commander Kim]

Say i were to build an AKMSU style krinkov with the 8.5 inch barrel. Would i be able to just cut down a 16 inch barrel, or would i have to find someone to make one for me with the proper twist rate and stuff. I'm fairly sure i can make the hand guard cuts and gas port hole and stuff. I just want to make sure I won't be wasting a barrel on a gun that will be key-holing at 10 yards. *cough* like the 5.45x39 does if you don't have the right the right twist rate and just cut the barrel in half *cough*

Sorry if this is a repeat, but i searched everywhere, but i din't find anything but "Just go with a yugo m92"

Thanks in advance.

[austin7.62]

Factory barrels were usually 1:9.5 I think. Most American barrels are 1:10. Maybe you can find a damaged barrel and have it cut down and re-crowned. Should work I think.

Also, 5.45 bullets may look like they keyhole at short distances, but from what I understand they can straighten out. You should be fine as long as you don't use a 1:9 barrel with the 5.45s :)

[Smith]

In 7.62x39 a 8.5" barrel is no different than a 16" as far as what is needed in twist rate etc. You may have to make some adjustments to the gas system but the barrel rifling is the easy part. Cut one down or buy one of the Yugo m92 barrels to make it work. Or you can just buy a n M92 and reconfigure it or leave it 1.5" longer. Which is what I would do.

Edited December 18, 2013 by Smith

[peejman]

Factory barrels were usually 1:9.5 I think. Most American barrels are 1:10. Maybe you can find a damaged barrel and have it cut down and re-crowned. Should work I think.

Also, 5.45 bullets may look like they keyhole at short distances, but from what I understand they can straighten out. You should be fine as long as you don't use a 1:9 barrel with the 5.45s :)

 

 

Right.  The 5.45 wobbles significantly in flight.  That's one of the reasons why the bullet causes so much tissue damage.  It's possible that the velocity is reduced enough with a short barrel that the bullet does indeed tumble.   

[Guest kingarmory]

Actually, the supposed "keyholing" or "wobbling" problem of the 5.45 round isn't due to the twist rate or the bullet generally.  This story became widespread several years ago when AK-74 and Tantals hit the market with US-made barrels.  The 5.45 round requires a .221" diameter bore but the US builders opted to use the more common .224" bore at the time.  After thousands of guns out there were having issues with crappy accuracy, keyholing bullets, and other bullet stability problems, the barrel companies started making US-made .221" barrels.

In regards to the OP's question, you should be fine, the 7.62x39 is a short and wide bullet that doesn't need too much speed to stabilize (its not terribly stable out of a full-length barrel, which is part of its notoriously poor accuracy).  For a barrel that length it would be optimal to run about a 1:8 twist, but the factory 1:9.5 will work fine.