I just rebarreled an upper with a 6X45mm barrel (preban--threaded) and I will be adding a threaded comp to the muzzle.
I have a number of different styles of compensators, and after checking the inside diameters (all were designed for 5.56/.223 rem) I find i.d.'s to be .255" - .275".
Is there a critical clearance spec for a bullet to pass through a compensator & have it still be effective (I know too big isn't going to work)?  The .243" bullet clears all of my comps easily by hand, but I was told there is a critical spec to follow.  Advise anyone?

I just went through all this with a friend doing the same, using a Cav Comp for 6mm.

Using the .255" with a .224" dia bullet as a minimum guideline, that is .255"-.224" = .031".  On the large end, .275" - .224" = .051".

So, let's take .243" and add .031", which is .271".  .243" + .051" = .294".  That gives you a range of .271"-.294" to consider with a .243" bullet.

To keep a similar clearance, you can drill the hole with a 9/32" bit, which is .28125".

I guess I can just duplicate the clearance in a .224 designed comp to be safe, but I was wondering if there was an "ideal" clearance that would give the most effective "braking".
I checked in some 'smith catalogs & most brands of "universal" muzzle brakes call for either .020" or .040" clearance.  That is a huge difference in brakes that are very similar in design!