I measured my levang comp muzzle opening and I got .245". Since the .223 is actually .224" that would be pretty close to the .020" you spoke of. I'd have it done on a lathe so you know the hole is dead nuts centered. Since you're 6mm is .243" you're looking to get as close to .263" as you can. I've had those 6mm-223 barrels on and off for about 18 years. All have been sub 1/4" barrels on a good day with handloads. Like an idiot I sold one about 2-3 years ago that was my most accurate one so far. I just got one from my buddy that is almost as accurate to replace it though.