New barrel or new upper?

[McGarrett]

I just found this; https://www.stagarms.com/20-5-56-heavy-barrel/

[Lumber_Jack]

How about this? http://www.gmriflebarrel.com/gm-m32-m-16-pencil-profile-20-long-1-12-twist/ or a 1:8 barrel I have found a few of these. Do I need a chrome lined barrel?

Man that is a super slow twist. I would hold out for a 1:9 or go 1:8. The 1:12 will probably limit out your weight in the 60gr area. Anything heavier won't stabilize. And that may be pushing it depending on bullets.

[shotgunshooter]

Man that is a super slow twist. I would hold out for a 1:9 or go 1:8. The 1:12 will probably limit out your weight in the 60gr area. Anything heavier won't stabilize. And that may be pushing it depending on bullets.

Is there really that big of a difference between a 1:8 and a 1:9? I am finding more 1:8 in 18".

[Lumber_Jack]

Is there really that big of a difference between a 1:8 and a 1:9? I am finding more 1:8 in 18".

If you're gonna shoot super light bullets it might. But I doubt you'd find much of a difference. Too fast of a twist and light bullets will come apart in flight. What's you're target range for bullet weight?

[shotgunshooter]

If you're gonna shoot super light bullets it might. But I doubt you'd find much of a difference. Too fast of a twist and light bullets will come apart in flight. What's you're target range for bullet weight?

Only planning on 55 gr and M193 for bullets and ideally 200-300 yds for my needs but I would like to go out to 500 a few times.

[Lumber_Jack]

55gr. You won't notice a difference between 1:9, 1:8, 1:7. That bullet weight is a middle ground that most barrels will stabilize just fine. Accuracy is dependent on barrel. You could take a 16" carbine with a 1:8 twist and shoot 2-300yds lights out with a good barrel. People shoot 600m with M4s.

1:8 has become more popular as heavier bullets became more popular. It's a great middle ground barrel

[shotgunshooter]

55gr. You won't notice a difference between 1:9, 1:8, 1:7. That bullet weight is a middle ground that most barrels will stabilize just fine. Accuracy is dependent on barrel. You could take a 16" carbine with a 1:8 twist and shoot 2-300yds lights out with a good barrel. People shoot 600m with M4s.

1:8 has become more popular as heavier bullets became more popular. It's a great middle ground barrel

So are you saying that it won't make that much of a difference? I just have this idea that I want a full length gas system.

[Lumber_Jack]

No a 1:8 would be just fine for what you want.

[shotgunshooter]

All of this is making me seriously reconsider getting a barrel.

[Ted S.]

55 gr is about the lightest you want to shoot with if you choose a 1:8. My go to ammo is 62 gr. My Barrel is 18", 1:8, with a midlength port and a gov't/socom profile (whatever the fuck that means; it seems like a slightly heavier than pencil profile to me).

[Dolomite_supafly]

I shoot M855 62g stuff out of my 12 twist. It is more accurate than my 9 or 7 twist barrels but you can see one side of the bullet hole isn't as sharp as the other side. All this means is the second they hit an organic target the bullets will tumble and hopefully fragment. My 12 twist shoots MOA using M855 while the others do not shoot as well. My SHTF AR has a 20" barrel so velocity is high enough that the bullets should fragment inside of 200 yards but even if they don't the bullet will still tumble causing damage.

 

An 8 twist barrel will shoot EVERYTHING that will feed from a magazine. 9 twist will shoot everything up to about 77 grains but all barrels are slightly different so verify before shooting through a silencer. With a 7 twist you are limiting yourself to ~50 grain, or heavier, bullets. Yes, you can shoot lighter bullets but some will sling the jackets right off the bullet with the faster twists. And there really is no need to use a 7 twist unless you are shooting 80+ grain bullets but once you get to about 80 they rounds can no longer be fed from a magazine and must be loaded one at a time.

 

8 twist is the best compromise between them all. The best way to figure out what twist you need is figure out which bullet you are most likely to be shooting and use the slowest twist that will stabilize it. In this case a 12 twist will be the best with 55 grain bullets but because you might also be shooting some heavier stuff or at longer distances I would go with a 9 twist. A 9 twist will let you shoot the most common stuff as well as the heavier stuff without the bullet being too stable. And this will ensure the bullets will tumble when hitting an "organic pop up target".

 

The wounding mechanism for FMJ bullets is tumbling and then fragmenting. First the bullet must tumble then, if the velocity is high enough, the bullet will fragment. Now if you have a twist that is too fast for the bullet then the bullet will not tumble, at least not inside the target, and in turn the bullet will not fragment. But if the twist is slow enough the bullet will first tumble inside the target and then the bullet will fragment providing the velocity is at least 2,500 fps. But it might require more or less velocity depending on the thickness of the jacket. Now even if the bullet is not going fast enough to fragment it should still tumble. If you shoot 55 grain bullets out of a 7 twist it is possible for the bullet to not tumble and pass through the target without doing a lot of damage like a 22 lr. This is why we are hearing all the reports of the 5.56 being ineffective against the thin middle eastern men. The way to fix that is to either go with a slower twist or a heavier bullet.

 

And the problem is not with the caliber as long as you realize it is a short range cartridge. It worked great until the bean counters decided having a 7 twist would be better for shooting tracers. Then after that the government decided to try to make this short range cartridge into a long range cartridge which, as we have heard, did not do so well. If they really wanted to fix the problems with a 5.56 they would accept the fact it is a short range caliber and either use a much heavier bullet or a much slower twist. As it stands right now the military is using the worst of both worlds, a twist that is too fast and a bullet that is too light for the twist.

 

In the end if I had to choose one twist for everything I might do it would be a 9 twist barrel. It is still slow enough that most bullets will readily tumble yet fast enough to stabilize most of the heavier bullets you might find.

 

Here is a pretty good read on what is going on overseas with the M855 and the faster twist barrels.

http://stevespages.com/pdf/5_56mm_military_info.pdf