Minimum - maximum I.D. Required of a brake?

[beltfed]

So the background story goes like this:
I recently had a couple of barrels threaded for muzzle devices and plan on running brakes on both.
The thin O.D. Of the barrels (0.590") meant the threads installed are a fairly common 1/2"x28.
One of the barrels caliber is 6mm (.243 win.) and the other .308 Win.
The 6mm barrel and muzzle device was easily sorted with an Insite Heathen I had for another build, but is admittedly designed for 6.5mm.
.308 muzzle devices as most know are commonly found in 5/8"x24 threads.
I could go with another Heathen for the .308, but poking around my spares bin I found a Smith Enterprises brake with 1/2x28 threads that, I believe was originally sold as an AR brake. The I.D. On the end mics at 0.345", so this has me thinking this SEI brake is actually for a .308 so should theoretically work.

Question: is the .345" opening going to be enough 'wiggle room' for .308, or am I going to blow off this brake in the first few/few hundred rounds? Or conversely, is it too much slop?
Along that line of thinking, is it possible the 6.5mm designed Heathen Offers too much slop as well? What is the accepted tolerance that would account for bullet diameter, pressure and possible turbulence, while still offering the best brake performance?
I'm reading on Snipers Hide and Accurate Shooter these guys aim for .2" overbore - but that is presumably a machined into the end of the barrel, integral brake and not an add on, threaded device (?)

Thoughts are appreciated.

 

[guntech]

If the brake is installed concentrically with the bore, it only requires 20 to 25 thou clearance on bullet diameter for maximum efficiency.

.2" over bore is 200 thou... way too much clearance... maybe a typo...

 

[beltfed]

Yes, typo. - 0.020”.
So the SEI that I have is 0.037” overbore on the .308.
Not ideal, but perhaps not too terrible. Only thing to do is give it a whirl.

 

[guntech]

Yes, typo. - 0.020”.
So the SEI that I have is 0.037” overbore on the .308.
Not ideal, but perhaps not too terrible. Only thing to do is give it a whirl.

Hopefully when installed it is concentric to the bore.

That is the one reason why I don't like pre threaded barrels and pre bored brakes. I much prefer to bore the brake concentric to the bore while it is installed and indexed.

 

[bearhunter]

Listen to guntech. He speaks the truth.

I like even closer tolerances when I'm building my own for personal use but then time isn't costly. On commercial or custom jobs time is money and the benefits from the tighter tolerances can be minimal.

One thing to keep in mind when brakes are installed. They need to be cleaned meticulously.

Just after Christmas a fellow brought me a lovely 6mm Remington with a very well made and well fitted brake. The work was done close to 35 years ago and the smith knew his stuff. The brake looked to be made/installed inhouse.

Now to the issue.

There was a rough lip around the interior edge of the muzzle. It was caused by hot gasses and powder residues building up and maybe even melting the metal from the lip of the brake. I couldn't remove it with solvents and a brush. I had to remove the brake, chuck it up and turn off the lip.

The brake had been there for a long time and required soaking and some careful heat to get it to budge.

I was worried it may not shoot well when it was re installed. I was lucky, it shot well. Sometimes these things can go awry no matter how careful you are. Cleaning was something the rifle wasn't used to by the way.

Also, adding a brake can cause some barrel harmonics issues that can be difficult to alleviate. Still, when installed properly they will be fine.

Maybe when you get back from Mars, you can get guntech to install your brake.

 

[beltfed]

The threading was done by CSC's gunsmith, and the brake is Smith Enterprises, so there are no concerns with the quality of the work or the muzzle device. Visually at least, they appear to be concentric to each other upon threading.
So with that in mind I made up a sudo bore gauge with brass rod, it appears to check out.

 

[JoKa]

random question but still somewhat inline....

You were running a brake for poops and giggles on your mkIV 22/45 Lite but if I recall that was causing cycling issues. Did you ever work that out?

 

[beltfed]

Nope. Took it off and it ran fine.
I think that particular brake was too effective for the little .22 blowback action on that Mk IV.
I have plans to try another on it, but sadly it’s taken a back seat as I’ve not been to a steel challenge this year

 

[bearhunter]

Nope. Took it off and it ran fine.
I think that particular brake was too effective for the little .22 blowback action on that Mk IV.
I have plans to try another on it, but sadly it’s taken a back seat as I’ve not been to a steel challenge this year

Try a lighter recoil spring.

 

[guntech]

Try a lighter recoil spring.

The brake is preventing the gases to escape as quickly as they are dispersed sideways... maybe it needs more spring?

Maybe diagnose it first. Will it function with low velocity? Will it function with high velocity? How does it malfunction?

 

[beltfed]

I had a spare AAC brake for an AR-15 kicking around and decided to try threading it on and see how it would run. It looks like a blaster from Star Wars with it on.
With low velocity (subsonic) .22 ammo it became a single shot. Zero recoil. I realize .22lr has next to none in the first place, but with this brake on it was akin to dry fire. With CCI velocitor it would shoot, but have failure to eject/failure to load in about 1 in 10, 1 in 20 rounds. The recoil is negative - as in the pistol is pulling from me slightly.
Also very 'blasty' - not much concussion, but did send it back to the shooter and quite loud.
It was a fun experiment but there are dedicated brakes for the Mk IV - I think for a reason.