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7.62X39 twist rates

jplooksterjplookster Member Posts: 96 ✭✭
edited June 2010 in Ask the Experts
Wondering what is typical twist rate for SKS, AK's and Ar-15's in this caliber. And why.

I have 2 Colt uppers a 16 and 20 inch that are stamped 1 in 12. Yet most other manufacturers 7.62X39 AR's are 1 in 10. (The 16 inch colt upper is very accurate sub MOA. I have not fired the 20" yet).

Curious about the method behind the madness.

Comments

  • Tailgunner1954Tailgunner1954 Member Posts: 7,815
    edited November -1
    Twist rate is determined mainly by the length of the bullet being used (longer bullet = faster twist), and to a lesser extent by the velocity at which it will be driven (lower velocity = faster twist).
    As the 7.62x39 mainly uses the shorter (read lighter) for caliber bullets, the slower twist rate will be more than acceptable.

    1:10 has been the "standard" twist for 30cal barrels since the days of the Krag with its long 220gr slug and fairly low velocities, but the 7.62 NATO (with it's 147gr bullet) went with the 1:12 back when it was adopted as a standard round.

    Note that most 7.62x39 rifles (including the SKS and AK varients) tend to overspin the bullet, leading to poor short range, but decent midrange, accuracy.
  • Hawk CarseHawk Carse Member Posts: 4,320 ✭✭✭
    edited November -1
    I figure the 10" twist (probably 240mm = 9.4" or 250mm = 9.8") of a SKS or AK is mostly due to the Soviets using the same tooling for 7.62x39 as for 7.62x54R and its heavier bullets. It is way more than needed for a 123 gr flatbase bullet, but was done for manufacturing convenience.
  • quickmajikquickmajik Member Posts: 16,324
    edited November -1
    I had always though 1-12" would be the best, but as said commie guns are 1-9.5"
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