A muzzle device, and its affect on the mechanical accuracy of a barrel

AR/M4 AR Technical Discussion
1

Bear with me on this one guys.

Considering the importance of a barrel’s crown and its ability to evenly disperse gases as the bullet leaves the barrel, so as to minimize initial yaw upon the bullet’s exit- how much can a muzzle device on an AR-15 barrel effect the mechanical accuracy of the barrel?

Granted, this issue may not come into play, or for all I know, even be measurable under, say 500 yards. However, I’ve been thinking about the topic lately, and have been told that I am not crazy-mental-insane for doing so. Therefore, I’ll throw the notion out here for a thrashing.

For example, take a known brand, higher-quality, AR-15 barrel that is finished with an 11° target crown. This barrel is also threaded for a muzzle device. Which muzzle device, if any, will allow the 11° target crown to do its job and have the least effect on the mechanical accuracy of the barrel? Conversely, how might the given design of a particular muzzle device affect mecahnical accuracy, if at all? Which muzzle devices might have the least affect, and which might have more of an affect?

I’ve searched and researched, and all I can find regarding muzzle devices is which ones hide flash the best, and which ones compensate or counter muzzle rise the best. Nowhere have I seen anything addressing how a device might affect accuracy. As for design - and this comes from viewing pictures online, not from holding each muzzle device in my hand - It looks as though some muzzle devices leave the barrel crown complete exposed (i.e. Ops Inc Brake), while others appear to almost completely cover up the barrel crown, and have their own internal crown (i.e. YHM Phantom line). For those models that have their own internal crown, do these snug up against the barrel crown when installed and properly clocked? If not, how big is the space between the barrel crown and the backside of the device’s internal crown, and what affect does this have on the bullet and escaping gases ? (hed esplode!)

This may not be a concern, as most of us probably a) don’t have high-end match-type barrel, and b) don’t have much opportunity to shoot more than 300 yards. In all honesty, I recently purchased a good barrel. I got to thinking, and wondered if I’d do the barrel a disservice by throwing on just any muzzle device. Chances are slim that this barrel won’t see an opportunity to reach out beyond 300 yards. But, pondering notions beyond the scope of reality can be fun and interesting, and sometimes I can take something useful away from the process.

Please weigh in with your thoughts and comments. Thanks.

2

Army Marksmanship Unit experience has been that a properly installed (there’s the rub) muzzle device has little to no effect on a rifle’s accuracy.

A device installed too tightly can put stress on the muzzle as it lengthens the threaded area, causing a “Trumpet bell” effect – weapon accuracy will fall off.

3

Hence the importance of installing a muzzle device with a torque wrench to mfg.'s specs and using a thread locker such as Rocksett. Your average Bubba the Builder will just crank it on like he’s installing a lug nut with a 4 Way.

Any muzzle device is going to change the barrel harmonics, which may in turn effect accuracy. But you’ll never know which one will or will not increase or decrease group size without trying it. It’s going to vary from rifle to rifle and attachment to attachment. No two are going to react the same.

4

I have heard that the AMU only hand tightens a muzzle device against a peel washer. The use of a peel washer also helps correctly clock thedevice.

It is interesting you mention barrel harmonics. In my travels on the interweb, I think I may have come across maybe two hiders/comps that metioned something about controlling, quieting, or dampening barrel harmonics.

5

Harmonics are a issue with accuracy. Even browning tried to get in on the action with their boss system. I think that was adjustable to tune for harmonics. If you do a search for .22 LR accuracy you will find in the competition guns a barrel tuner is often used. The same with a bughole tuner that puts pressure on part of the barrel. There is even yet another method that moves a pressure point along the barrel. The kicker with tuning for harmonics is it changes with the ammo. Say you tune it for one and its likley it wont shoot another the same.

As for brakes and flashiders i wish i could shoot good enough to know. It seems like everytime ive tried to test that it just doesnt work out. Ive wondered how can a brake or a flashider with that first baffle not hurt accuracy. Thats nowhere near a crowns accuracy and there is a small high pressure spot the bullet has to pass through right in between the crown and that baffle. Maybe it all happens so fast thats why they say its a nonissue.

One thing i do know is that having a barrel reprofiled and or threaded can effect accuracy (harmonics). Dont ask my why but i had a stainless 16" barrel that shot great turned to a custom profile including a M4 notch :rolleyes: and when i got it back the barrel wouldnt group near as well with the same ammo. That was even without a muzzle devise on.

I guess the big question is… Is the air so less dense in front of the bullet that by the time the bullet has hit the trouble spot all there is to effect is the pressure behind it? That would explain why brakes wouldnt have much of an effect since they arnt really breaking a seal with a bullet. Those holes are large enough that the bullet isnt bothered by them. The crown is so important because you dont want to unevenly break the seal on the bullet and caue it to yaw in a direction other than straight.

6

Great Info!

7

In other words an improperly installed muzzle device can be detrimental to accuracy.

Improperly can mean = overtorqued, too lose, non-concentric barrel threads, out of round muzzle device, carbon build up in muzzle device, unsquare shoulder on barrel behind threading. I’ve also seen crash washers do some strange things which is why I don’t use them on rifles with magnified scopes. On these I only use precision indexing washers or peel washers or better yet have my barrel turned so that the flash hider/comp times up with no washers of any kind.

8

Damn… never even knew that was possible, man. Do you basically tighten the muzzle device down first, then pull the barrel out of the upper, turn it until the muzzle device is properly aligned, and then put it back together again?

9

I recently installed a brakeout from aac. My POI was 4" low at 50 yds after installation. Is this to be expected when changing muzzle devices?

10

If you’re going from a flash hider to the Brakeout, which is a hybrid flash hider/comp it’s not surprising. The real question is, what difference did it make in group size?

11

Not much that I could tell at 50 yds. I haven’t had a chance to shoot it at 100 yet.

12

My major first hand experience with a difference in muzzle devices was between a standard A2 and FSC556.

First started with a BCM middy, 12" lite rail, used an Aimpoint to zero with AP and A2 zeroed at 100 and remember having about 2" group, average.

Talked into a FSC556, put it on and I was 5" off at 25! low and right. I zeroed it and later got a Nightforce 1-4 w/CQB reticle. I could not do better than a fat 3" at 100

I put the A2 back on and zeroed at 100 with the scope–moa

All three times muzzle devices installed with crush washers, new ones. All ammo was 55 grain ball.

That’s my experience with impact shifts and accuracy changes. Nothing scientific, for or against a product, etc

13

Pretty sure he means “turned” as in on a lathe. A properly-equipped machinist can remove material from the barrel, in effect lengthening the distance from the barrel shoulder to the threads in such a way that when the muzzle device butts up against the shoulder, it will be indexed in the correct position.

14

The front edge of the barrel is shortenend a tiny bit to allow the muzzle device to line up correctly with no washers (crush or peel). My Benny Hill Rolling Thunder is that way.

15

Well shit… I feel like a douche-bag now:p Although as I read crazymoose’s comment above, I instantly remembered reading in my surefire suppressor attachment instructions that the “optional” method for gunsmiths is to do just that. I just completely forgot about it… even though it was only 2 weeks ago.:smiley:

Thanks for the clarification though, guys!

Semper Fi,

-Paul

16

Funny, I installed one of those about a month ago, that’s how I learned about it. After dicking around with those shims for over half an hour trying to get the torque just right, only to find that applying Rocksett seemed to alter the timing when it had been lined up perfectly, I think the machining method might actually be easier to someone with the skill and tools.