AR15/M16 Barrel

Discussion in 'AR-15 Discussion' started by JonM, Dec 27, 2010.

  1. JonM

    JonM Moderator

    for those that may know some of the more grognard history of the M16-A1 i got a question

    1in12 or 1in9 twist rate with 55 grn fmj.
    what is more effective the moment the bullet hits the target as in destablizing and the bullet taking wierd paths or going to bits?? max effective range 300yards.

    need enough accuracy with fmj to hit yote sized targets at 300 yards.

    which twist rate will get a 55grn fmj there. what was the issued twist rate in the M16-A1 in the period from 1967-1980 when they started issueing the A2 with the 1in7.

    ive come across a lot of conflicting info on this.
  2. willfully armed

    willfully armed New Member

    1:12 is better for lighter "varmint" bullets.

    1:9 is good for 300 yards with 55gr. But, why FMJ on an animal?

  3. Jpyle

    Jpyle New Member

    M16-A1 was a 20", 1:12 pencil barrel I believe. Either 1:12 or 1:9 will handle a 55gr...1:12 gives you more options going down in weight...1:9 going up.
  4. Quentin

    Quentin New Member

    Interesting question and I hope you get a lot of replies.

    I'm no expert but from what I've read 1:12" was just enough for a stable flight and accuracy but gave a "poison bullet" effect where the bullet destabilized quickly upon striking the target. Upon entering the body it would turn and split in two or fragment, creating a much larger wound cavity (cavities) than say the 7.62x39 in Vietnam.

    That effect is lessened in 1:9 and certainly 1:7 but of course today heavier/longer 5.56 bullets are used in the military.

    Anyway, tough call as far as choosing your barrel and I guess the best bet is to try out different twist rates with 55gr and at different distances up to 300m.
  5. JonM

    JonM Moderator

    not gonna use it on a beastie just using that as a refernce point as in hitbox size. i just want it accurate enough to hit an area the size of a coyote at that distance.

    im trying to recreate the look and charateristics of the old M16-a1. i like old school stuff
  6. orangello

    orangello New Member

    edited* Have you seen that old school kits like they had at I don't see it on their sight now, but i'm sure it was a century deal. re-edit** The non-kit Century model.
    Last edited: Dec 27, 2010
  7. Sniper03

    Sniper03 Supporting Member

    The M-16 A- Rifles were 1:12 Twist. The 55gr. Bullet will also shoot decent in the 1:9 the 1:9 will also handle 60s to low 70gr. bullets.

    Here is a little formula to play with regarding the difference required in bullet rotation to stabalize a projectile. The heavier the projectile the faster it must spin (RPM) to stabalize in flight. If you shoot 69+ grain bullets out of the 1:12 you will normally get some key hole and side strikes because the bullet is not spinning fast enough.

    Here is a little formula for you to have some fun with.
    Velocity of the bullet X 12=___________ Divided by Barrel Twist of (12)=___________ X 60 = The RPM of the projectile

    The first *12 = Inches and the *60 which equals minutes is always in the formula. Velocity can also change the formula.

    Example 1:12 Twist

    3000 fps X 12 = 36000 Divided by 12 = 3000 X 60 = 180,000 RPM (Max 55gr.)
    FPS IN. Twist Minutes

    The same formula above for the 1:7 Twist RPM=308,571

    So you quickly see that the heavy bullets like the 69gr and 70+ bullets require a much faster twist to stabilize. The 1:12 is too slow. I have also seen an incident where a 42 gr HP Varmint bullet fired in a 1:7 never hit the target at 100-200 yards evidently coming apart due to RPM figured on the minimum of the 1:7 twist at 308,000 RPM + It is interesting to check this kind of thing out! Makes ballistics interesting!
  8. JonM

    JonM Moderator

    yeah ive seen both of those the century is a stainless barrel. im more looking for parts rather than an entire rifle. im still kicking myself for not buying a half dozen surplus barrels when you could get em for 50$ a pop.
  9. JonM

    JonM Moderator

    damn!! ive never seen that formula before!! then that means im looking for the spare to be a 1in12 cuz i will definately want to run the light varmint rounds out of it on occasion.

    now just gotta find somone that makes a slow twist barrel.
  10. Jpyle

    Jpyle New Member

    Contact JD, he had a Colt A1 1:12 pencil barrel in the upper his shop built for his buddy that eventually made its way to me...:p
  11. JonM

    JonM Moderator

    gonna have to see if he still has it
  12. Jpyle

    Jpyle New Member

    I have it now but he should know where it came from.