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Old 02-20-2011, 11:45 AM
ArmAtWill ArmAtWill is offline
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Default Different 5.56 grains for different rifling twists?



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I know there are different grain bullets for 5.56mm like 55, 62, 72 etc and I know that the 55g was the original and AR rifles were made with a 1:9 twist barrels for them. I also know that people are now using heavier bullets and manufacturers are starting to produce 1:7 twist for the heavier bullets. My question is what grain bullet (and up) is too heavy for the 1:9 and should be used in a 1:7 barrel?
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Old 02-20-2011, 04:21 PM
wmbanse wmbanse is offline
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Default Bullet weight

you are correct, you should match your rifles twist to the wieght of the bullet.

as a point of history the ar-15 was designed with a 1 in 14 twist, then after testing in cold weather it was tighted to 1 in 12 (with the adoption of the m16) to improve accuracy. in the early 80's when the m16a2 was adopted they tightened the twist again to improve accuracy due to adopting a heavier (64 Grain) tracer bullet (remember a tracer will be longer than an equivalent ball bullet of the same wieght).

now there is a formula to calculate "ideal" bullet twist. the Greenhill formula

Http://www.google.com/url?sa=t&sourc...sXKDD8e3Lgra7Q

you enter in your info and this site does all the work.

as for the twists you list a 1 in 7 will stabilize bullets that you are forced to load one at a time they are so long (80+ grains). however the down side to having a over stabilized bullet is two fold. you could have bullets that come apart in flight due to centrifugal force. also part of the wounding effect that was observed in early tests of the ar-15 was due to marginally stabilized bullets tumbling when they hit anything solid.

a 1 in 9 twist is in general a great compromise and i have used it with bullets from 55-75 grains with great success.

in the end a lot of it is personal preferance.

i hope this helps.

v/r
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Old 02-20-2011, 08:11 PM
andersed andersed is offline
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Quote:
Originally Posted by ArmAtWill View Post
I know there are different grain bullets for 5.56mm like 55, 62, 72 etc and I know that the 55g was the original and AR rifles were made with a 1:9 twist barrels for them. I also know that people are now using heavier bullets and manufacturers are starting to produce 1:7 twist for the heavier bullets. My question is what grain bullet (and up) is too heavy for the 1:9 and should be used in a 1:7 barrel?
it is not just the weight of the bullet, but the length. Remember a M855 (green tip) round is 62 grains, but it will be longer than a lead core FMJ round of the same weight. Generally, 1/9 twist will stabilize just about anything you shoot up to 77 or so grains. The exception to this is a M856 tracer round as noted above, you need a 1/7 twist rate for that.
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Old 02-23-2011, 05:43 PM
ArmAtWill ArmAtWill is offline
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Thank you very much that twist calculator is very helpful!
Old 02-24-2011, 02:16 AM
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Quote:
Originally Posted by andersed View Post
it is not just the weight of the bullet, but the length. Remember a M855 (green tip) round is 62 grains, but it will be longer than a lead core FMJ round of the same weight. Generally, 1/9 twist will stabilize just about anything you shoot up to 77 or so grains. The exception to this is a M856 tracer round as noted above, you need a 1/7 twist rate for that.
Performance of 1-9 twist barrels with 75 and 77 grain bullets is hit and miss, and varies from weapon to weapon.

If you plan to shoot a lot of stuff over 62 grain, the USGI 1-7 or 1-8 are going to be better bets. If you've already got a 1-9 the runs 77 grain stuff accurately, no harm no foul, but if you're buying something new be aware it might not work well with heavier rounds.
Old 02-24-2011, 09:46 AM
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FYI,the earlier AR's used 1-14 twist barrels.Most of them without a forward bolt assist have the slower/better twist.

..I say better because the slower twist created much more devastating wounds as a result of the 55 gr.bullet just barely being stabilized.
Old 02-25-2011, 07:36 PM
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I would argue that a 1:9 will stabilize a 77gr bullet.

Every barrel maker is different, so you can have two barrels that have a 1:9 and one will stabilize a heavier bullet, while the other barrel will not stabilize it.

Only way to know for sure is to buy some factory loads in the weight you want to shoot and test it in your barrel. If your barrel will stabilize the bullet...then buy in bulk.

If I was looking for an AR15, I'd get one in a 1:7 twist since it will shoot 55gr and up...you will have the option to use the heavier 75/77gr bullets.
Old 02-26-2011, 01:32 PM
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Originally a 55 grain 5.56 was fired out of a 1:12 twist barrel (the 1:14 was deemed too unstable by NATO); 1:7 twist was introduced for the 62 gr. bullet, which the change to a heavier bullet was over being able to have a reliable tracer round that had a close enough trajectory from the ball ammo (the 55 gr. and tracer previously did not have the same trajectory and therefore the traces was essentially useless).

The 1:9 twist is a civilian invention that is meant to cut the difference between the 1:12 and 1:7 twists and essentially is meant to be a decent all around twist - which, actually a 1:9 twist is the ideal twist rate for a 62 gr. bullet.

I use 1:7 twists on my AR barrels, I had a 1:9 and didn't like it. I shoot 62 and 77's mainly.
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