Gunsmithing Machining a muzzle break bigger?

[MTS308]

I have a APA little bastard gen 2 muzzle break in 6.5 mm that I'd like to open up to use on a 308win.
I've read somewhere I need to open the bore I.D up .020 to .030 bigger than the bullet diameter. Which should make the bore .328 to .338 I.D. Does this sound right?

 

[theLBC]

i don't know what the rule or standard is, but my surefire measures about .325"

 

[FisherT&C]

.328 will be plenty

 

[MTS308]

Thanks

 

[atvman400]

.328 will be plenty

As long as the treads are concentric to the bore. If they are not, .328" might not be enough.

 

[theLBC]

As long as the treads are concentric to the bore. If they are not, .328" might not be enough.

i think that is the main thing...avoiding a strike.
the smaller the hole, the more effective the brake, but somebody tested a .308 brake on a 6.5CM and it didn't reduce effectiveness drastically.

 

[MrSmith]

This isn't the question asked but, as an aside, I would recommend making sure you have a quality, correctly angled, sharp drill bit. The brake I opened up was surface hardened and took some effort to get started correctly. After through the hard zone it cut easy.

I have heard that effectiveness starts to fall off at around 60 thousandths over bore diameter. I don't have the knowledge to know if this is accurate or not. I went 30 thou over. And the brake was drilled and re-threaded on the same setup.

Thank you,
MrSmith

 

[MTS308]

This isn't the question asked but, as an aside, I would recommend making sure you have a quality, correctly angled, sharp drill bit. The brake I opened up was surface hardened and took some effort to get started correctly. After through the hard zone it cut easy.

I have heard that effectiveness starts to fall off at around 60 thousandths over bore diameter. I don't have the knowledge to know if this is accurate or not. I went 30 thou over. And the brake was drilled and re-threaded on the same setup.

Thank you,
MrSmith

I will probably use a boring bar with a carbide insert so it shouldn't be a problem. I have 2 I'm wanting to open up 1 stainless and 1 black nitrided. Probably try the stainless first.

 

[Wannashootit]

I use reamers to open them up, I snapped off a couple of skinny solid carbide boring bars due to the interrupted cuts and just bought a handful of reamers of the correct sizes. Much faster than boring them out as well...

I know if I run a .330 reamer into it, that's exactly what the hole will measure, all the way through.

 

[Vodak]

If you have the ability to dial it in a four-jaw or have a good collet system you can go as low as .010" over. But you need to verify that it all threads up concentric.
Honestly I think you'll be better served trading them for .30 cal brakes than taking on the project, but I totally understand the desire to do the work yourself.