Military M4 vs. AR-15 ?

[PittsburghOzzy]

New AR owner (S&W M&P 15, Chambered in 5.56),

Have a question concerning ammo grain. Having used 77 gr 5.56 ammo when in the military with a 1:12 twist(I think) with a 14 1/2 inch barrel, are there any problems? malfunctions? or anything I should know about attempting to fire a 77 gr .223 through the MP 15 which is a 1:9 twist in a 16 inch barrel?

Any help is greatly appreciated.

[saleen322]

New AR owner (S&W M&P 15, Chambered in 5.56),

Have a question concerning ammo grain. Having used 77 gr 5.56 ammo when in the military with a 1:12 twist(I think) with a 14 1/2 inch barrel, are there any problems? malfunctions? or anything I should know about attempting to fire a 77 gr .223 through the MP 15 which is a 1:9 twist in a 16 inch barrel?

Any help is greatly appreciated.

A 1 in 12 is a pretty slow twist for a 77 grain round. It may well have been a 1 in 7 or 1 in 9. The 1 in 9 will work for 69 grainers but too slow for 80. I never used a 77 in a 1 in 9 but you are kind of on the edge.

[Policraticus]

1:9 is a bit slow for such a long projectile. I might be wrong but the 1:9 is for the 62gr projectile. Stability and accuracy will suffer if you fire 77 grainers.

[ungawa]

For 77 grain you probably need 1/7. 1/8 might do, but it's less common and I know less about it.

1/9 usually will not shoot 75 grain accurately. Hornady TAP 75 grain is supposedly an exception for 1/9.

[Suburban]

1:9 is a bit fast for such a long projectile. I might be wrong but the 1:9 is for the 62gr projectile. Stability and accuracy will suffer if you fire 77 grainers.

Nah. Longer bullets need faster twist.

77 grains may or may not shoot well with a 1:9" twist, but you may be okay. 1:8" or 1:7" would be a safer choice for the heavier/longer bullets.

I think 1:14" was what they used for the first M-16s for 55gr bullets, and then it was toned down a bit to 1:12". 1:9" and heavier barrel for the A2 and 62gr bullets and National Match shooting.

[JayBell]

1/14" twist, 40gr or less - original AR twist, bad in cold environments in military arctic tests

1/12" 55gr - original military twist used until the A2 models

1/9 " 62 and 55gr does neither the best, but adequate for heavier loads

1/7" twist. Changed to in the mid 80s as the 62gr tracer caused excessive wear on the 1/12" twist. Stabilizes 62 and greater, up to 80gr single loads . Will "overspin": 55gr ammo.

[EvoRich]

75gr ammo through my 1:9 M&P15 groups just fine. The zero is obviously going to be different out to 100 yards from lighter ammo. But if you zero for it, it'll shoot that far just fine.

At 25 yards, you'll hardly notice a difference.

1:7 twist may be optimal for a 75-77gr bullet, but 1:9 can handle it, atleast in my experience.

Of course, the 75gr rounds I was shooting was TAP. There is obviously truth in key-holing and inaccuracy when using heavy weight bullets for slower twists, but I think people blow it out of proportion.

Only way to tell is buy a box or two and try it out.

[Lycanthrope]

75gr ammo through my 1:9 M&P15 groups just fine. The zero is obviously going to be different out to 100 yards from lighter ammo. But if you zero for it, it'll shoot that far just fine.

At 25 yards, you'll hardly notice a difference.

1:7 twist may be optimal for a 75-77gr bullet, but 1:9 can handle it, atleast in my experience.

Truth.

It isn't just TAP. My 1:9's shoot Nosler 77gr's just fine.

Lycanfearnotthrope

[The_War_Wagon]

Factory barrels are a lot like factory Chevrolets - NEVER buy the model made on Monday morning or Friday afternoon, because while they might LOOK the same, that loose rattle in the doorpanel could well be a crumpled beer can.

Accordingly, SOME 1: 9 bbls might actually be anywhere from 1: 8.7 - 8.99, with the lower twist rate handling heavier bullets better. Of course, some might LIKEWISE be closer to 1: 9.2 twist, and even 69gr. bullets could start keyholing!

If your 1: "9" bbl handles the heavier bullets well, you have a GOOD one, even if it is technically out of spec towards a 1: 8 twist. If you have trouble with 69gr. bullets, well... there's always a NEW rifle out there waiting for you!

[Lycanthrope]

Has anyone here had a 1:9 barrel that wasn't chromed keyhole 62 or 69 gr?

I've heard of it, but over decades of rifles, I've never seen it with my own eyes.....and I've put a lot of heavy rounds through more than a handful of 1:9 guns.


Lycananyone?thrope