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1:7 better for fragmentation?

Discussion in 'Black Rifle Forum' started by G26man, Jan 4, 2011.


  1. G26man

    G26man
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    I'm reading a novel in which the author works several historical incidents of shootouts/raids, etc. by the federal government into the story line. The incidents are recanted in great detail, and he was pretty darn accurate on the only one I know any details of (the FBI Miami shootout).

    He states that during the Waco raid the defenders on the ground level were shooting AR-15s up through the floor at FBI agents on the second story. He says that because the rifles had 1:7 twists and they were shooting standard 55gr ammo the bullets would fragment quickly and did not penetrate the floor as well as they would have had they been shooting a lower twist rate (or of course, heavier bullets).

    I'm curious if that is true historically (were they really shooting 55gr rounds through 1:7 barrels?) but what I really want to know now is does this theory have any validity? Has anyone done gelatiin testing on lightweight rounds (55gr or under) at 1:7 and do they fragment sooner upon entry than the same rounds at 1:9, for example? I'm guessing that this would have to be true at least somewhat. If you have a lightweight round spinning just under the point at which it will fly apart in air and it contacts a denser medium, it seems to me that it would fragment pretty quickly, although I admit I'm just speculating. Anyone know of any data to back this up?
     

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  2. faawrenchbndr

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    I call BS,........a 1/7 barrel will stabilize 55 gr ammo just fine.
     

  3. RMTactical

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    I agree.
     
  4. G26man

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    Sure it will. In air. I've seen someone state here that he got outstanding groups with 40gr vmax out of his 1:7 barrel. The question is does rotational velocity have an effect on fragmentation in gelatin or flesh.

    It's my understanding that the effect of too fast a spin will not destabilize a bullet but cause it to fly apart fairly soon after it exits the barrel. So if the bullet is spinning very fast, just under what would be required for it to fly apart in air when it hits someone or a gelatin block, what happens? Does it behave the same as if it was spinning slower, or does it fragment sooner?
     
    #4 G26man, Jan 4, 2011
    Last edited: Jan 4, 2011
  5. MisterPX

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    For 55gr with a cannelure, velocity is what determines fragmentation.
     
  6. cowboy1964

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    Rotation is not velocity and vice versa.
     
  7. MrMurphy

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    55 gr stabilizes in 1/7 fine, it's just not as accurate at longer ranges from what I've been told.

    Never had to shoot 55's out of one past about 100 yards.

    One of my former teachers was one of the snipers involved in the Waco incident at the end, but he wasn't there at the beginning. I'll throw a few questions his way.
     
  8. K. Foster

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    That’s wrong. Fragmentation is a combination of velocity and bullet construction.
     
  9. faawrenchbndr

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    A faster twist, say 1/12 will destabilize the projectile and make it tumble upon impact.
    That will accentuate fragmentation. But, fragmentation is about bullet design.
     
  10. GenoTac Ind.

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    I thought 1:12 is a slower twist? Well, slower than 1:7 and 1:9. To my understanding, bullet twist, rotation, spin, is for flight stability. Fragmentation would depend more on bullet speed, velocity. Try shooting in water with the same bullet but different velocity with the same barrel twist rate. Faster bullet would fragment more than the slower bullet. I think an episode from "Myth Busters" proved a portion of this.

    "The rate of twist, expressed as one turn in so many inches (i.e. 1 in 10"), is designed to stabilize the range of bullets normally used in a particular caliber. It takes less twist to stabilize a given bullet at high velocity than at low velocity. At the same velocity in the same caliber, longer (pointed) bullets require faster twist rates than shorter (round nose) bullets of the same weight and heavier bullets require faster twist rates than lighter bullets of the same shape. It is undesirable to spin a bullet a great deal faster than necessary, as this can degrade accuracy. A fast twist increases pressure and also the strain on the bullet jacket."
    -The Rifle Barrel; By Chuck Hawks
     
    #10 GenoTac Ind., Jan 5, 2011
    Last edited: Jan 5, 2011
  11. faawrenchbndr

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    Oops,.......yep it's slower.
     
  12. fuzzy03cls

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    BS. The twist rate should have little effect. The bullet & how it's loaded would be a bigger determination. You can not say it was anything with the rifle until the ammo used is identified.
     
    #12 fuzzy03cls, Jan 5, 2011
    Last edited: Jan 5, 2011
  13. Gunnut 45/454

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    It is well documentated that the 55 gr bullet will come apart in close range barriers! Think about it max Vel hitting hard barrier Floors, joices, 2x6's etc. They will frag! With 55 FMJ the best impact distances are 50 yards to 200 yards! After 200 yards 55 FMJ may or may not frag! I also heard /read that the original 1:12" twist actually helped frag at closer ranges!:whistling:

    Remember the whole point of FMJ is to wound -not kill! Thats why the Geneva Conventions were done after WWI! Cause we actually had bullets that killed before then! Think hunting bullets, nice soft lead bullets! Big nasty wounds!:whistling:
     
  14. crazymoose

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    I agree with the others. Fragmentation is a function of the bullet yawing in tissue, and the forces involved in that yawing being great enough to break the bullet apart.

    A modern bullet travels "backwards" with regard to its center of gravity. The spin is what keeps it from tumbling end-over-end as it flies. Once the bullet enters a denser medium (like human tissue), it would require a far higher spin rate to remain stable, and thus it yaws (and potentially fragments). If anything, the faster twist will make the bullet slightly less prone (probably almost imperceptibly so) to yaw and fragmentation.
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  15. cyrsequipment

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    That wasn't the geneva convention....

    http://www.thegunzone.com/hague.html
     
  16. humanguerrilla

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    +1 fragmentation more about shooting 55gr bullet right into ceiling rather than the twist rate out of full length a2s.
     
  17. DWARREN123

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    The bullet construction not riflle twist rate determines fragmentation.
     
  18. Alderak

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    I got to go on a trip to a nice long 800yrd backyard range around Christmas time and I was hitting an oil drum at 500 yards all day long with my 1:7 twist barrel and 55gr federal bulk pack ammo.

    Just hitting the barrel isn't exactly pinpoint accuracy, but I'd call it good 'nuff.

    I can't speak to the fragmentation though, never shot anything alive with my ar except gophers... and they don't leave much evidence.
     
  19. surf

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    Velocity and bullet design determine fragmentation. As velocity in flight decreases chances of bullet fragmentation also decreases. Also the medium through which the bullet travels also has an obvious affect.

    1:7 is faster than 1:12 and twist will determine the projectiles rotational stability in flight and will more directly affect accuracy. A heavier bullet tends to need more twist.

    55gr out of a 1:7 will work well. However I get better accuracy results out of a 75-77 gr bullet in a 1:7. This accuracy difference is small when considering a battle rifle, but it does exist.

    The Geneva Convention did not define bullet type for use in combat. It was the first Hague Convention of 1899, particularly Declaration III which outlined the Use of Bullets Which Expand or Flatten Easily in the Human Body.

    I have yet to see a twist rate that could cause deformation and / or fragmentation of a 55gr bullet in flight.
     
  20. WhiskeyUnicorn

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    I got a 1:7 to stabilize heavier bullets...but it slings my 55gr down the way no problem either. I think it's all in the way the bullet was built
     
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