Stupid question: .308 vs 7.62x39

[Overwatch326]

Just wanted to put this out there to confirm: I'm guessing a 7.62x39 muzzle brake cannot accommodate a .308 unless the device is specifically labeled to be ".30 Caliber"? My calipers are busted, but according to wikipedia, there's several thousandths' difference in bullet diameter, right?

 

[RRomig]

If threads are good your good

 

[Muleskinner]

Some 7.62X39 bullets are .311 instead of .308 but I doubt your muzzle brake is .308 I would get a new caliper and measure

 

[alucard0822]

.308 vs .311. Most 30 cal muzzle brakes and suppressors have a bore diameter around .350", so they will work. Problem is the muzzle threads, most euro stuff will be metric, 18x1, or in AKs 14x1mm with left handed threads, US 308 stuff is 5/8x24. AKs are really hard to get concentric threads for a can, a shorter/lighter brake will be OK. Some adapters also have to torque against the crown being an AK's thread shoulder is usually the sight base. Some things like 24mmx1.5 are actually a threaded sight base, and adds to the likelihood that it won't be concentric for a can. Of course a muzzle device in the original threading should be kinda easy, but most use a detent and just thread on a little loose, and can wobble or brush the side of a longer brake and cause accuracy issues.

For my suppressed Yugo M70 I had to put a concentric 0 degree crown on it first, then install and rockset the 5/8x24 adapter with clearance to the FSB, it seats to the muzzle. Then check bore and concenticy to make sure it didn't cause any issues compressing the muzzle. Shimmed and torqued a 5/8" 30cal Griffin taper mount, and tested concentricy again. with all being OK ran it with my rifle rated 9mm can, and rechecked everything, all OK. This project had a lot more difficulty than just another suppressed AR, but it works fine and is hella fun. Still the tiny contact area of the muzzle that a muzzle device and suppressor adapter seats to makes me nervous, so I just use 9mm cans with it, lots of obstacles being the muzzle after crowning is really the only concentric thing on an AK.

View attachment 337228

View attachment 337229

 

[camo556]

30 cal/308 muzzle brakes are oversized (usually by 2-3 hundredths at least)**. If it threads, send it. I have. However, its a good idea after the muzzle brake is on to check bore alignment/concentricity. Borrow someones bore alignment rod or buy one (you will use it a lot). There is a small chance that the threads are not concentric. Like a 99.5% chance you are fine and can just look down the unloaded barrel to check clearance, but the 0.5% is a bitch.


** Also while some 7.62x39 is .311, many American-made or NATO-made barrels are actually 308 because they use the same blanks for 308, 300 BLK and 7.62x39 barrels, which will squeeze the bullet to 308 anyway.

 

[Overwatch326]

Okay, so I double-checked the measurement with my calipers (battery died when I did the measurement before), and it came out at .3490", so I guess it's fine; just got a little nervous since it says "for 7.62x39 only" all over the packaging, wonder if that has more to do with ballistics. I tried threading it on, and it fit tight, but it lines up with the detent perfectly, so I suppose I'm good to go.

It figures that the Manticore Arms "Nightbrake" I ordered a couple months ago came off backorder and shipped while I was fiddling with this; I meant to cancel it. The nice thing with the VG6 Epsilon is you can get a blast shield that clamps over it, so the guys at the range don't hate me, but we'll see how the Nightbrake fits, too; maybe I'll like it better.

 

[Overwatch326]

Okay, so I double-checked the measurement with my calipers (battery died when I did the measurement before), and it came out at .3490", so I guess it's fine; just got a little nervous since it says "for 7.62x39 only" all over the packaging, wonder if that has more to do with ballistics. I tried threading it on, and it fit tight, but it lines up with the detent perfectly, so I suppose I'm good to go.

It figures that the Manticore Arms "Nightbrake" I ordered a couple months ago came off backorder and shipped while I was fiddling with this; I meant to cancel it. The nice thing with the VG6 Epsilon is you can get a blast shield that clamps over it, so the guys at the range don't hate me, but we'll see how the Nightbrake fits, too; maybe I'll like it better.

Welp, just heard back from VG6 themselves, and they said it's only for 7.62x39; wish the guy had explained why, but if the manufacturer's saying it, guess that's the final word on the matter.

 

[alucard0822]

Welp, just heard back from VG6 themselves, and they said it's only for 7.62x39; wish the guy had explained why, but if the manufacturer's saying it, guess that's the final word on the matter.

what rifle and what brake?, the muzzle dia will be about .350 on either, and not be an issue between the .002" difference in bullet diameter. Doesn't matter if it's 7.62x39, 7.62 nato, 7.62x54R or 300BO. The actual difference is muzzle threads, and that could potentially be an issue, sometimes you can crossthread a 5/8" brake on a 14X1LH AK muzzle and damage it. Ideally if it's an AK with 14X1LH threads, you need an adapter for 5/8" 308 brakes or an AK brake with 14x1 threads. In either case the bore diameter isn't an issue.

 

[Overwatch326]

It's a Zastava M77 in .308, the brake is a VG6 Epsilon AK 7.62x39, threaded for 14x1LH. I know it's probably fine, but if the manufacturer's saying not to do it, that gives me pause.

 

[camo556]

It's a Zastava M77 in .308, the brake is a VG6 Epsilon AK 7.62x39, threaded for 14x1LH. I know it's probably fine, but if the manufacturer's saying not to do it, that gives me pause.

Likely for warranty or liability reasons. The brake on a 308 will be fine, but possibly might wear faster. I am not saying it will wear faster. I am saying maybe they didn't test it, and its possible, so they disclaim it. If it has a coating (anodization or cerakote) the coating could wear faster. What is the brake made of?

Edit: to notice the difference in wear, you probably have to run it full auto, get it really hot, for thousands of rounds. The powder in 7.62x39 is generally burnt by ~13 in. whereas in some 308 loads powder will keep burning to 20"+. This can cause wear, in addition to higher pressures.

 

[alucard0822]

It's a Zastava M77 in .308, the brake is a VG6 Epsilon AK 7.62x39, threaded for 14x1LH. I know it's probably fine, but if the manufacturer's saying not to do it, that gives me pause.

You are good to go, the only spec issue is threads, as long as they are OK, and it's tight enough that it doesn't wobble, then have fun, will probably never have an issue with it. Most manufacturers don't really redesign brakes for specific calibers might be a little less efficient if it's got port sizes designed around the lower pressure/volume of 762x39. As far as wear or damage, not going to make a difference. They probably do get asked if their 308 brakes work on AKs, or the other way around, the answer has more to do with the barrel threads than anything else.

 

[Overwatch326]

You are good to go, the only spec issue is threads, as long as they are OK, and it's tight enough that it doesn't wobble, then have fun, will probably never have an issue with it. Most manufacturers don't really redesign brakes for specific calibers might be a little less efficient if it's got port sizes designed around the lower pressure/volume of 762x39. As far as wear or damage, not going to make a difference. They probably do get asked if their 308 brakes work on AKs, or the other way around, the answer has more to do with the barrel threads than anything else.

Okay, guess I'll send the Nightbrake back, then; I'm glad the VG6 seems like it'll be fine 'cause the Nightbrake wobbles like crazy, but the VG6 is nice and tight, even without the retaining plunger.