I thought I'd share an experience I had with muzzle brakes.
A little while back, I threaded a rifle barrel for a muzzle brake, intended for use with a suppressor. I indicated the barrel on my lathe, using an insert in the muzzle to ensure the threads were cut concentric to the bore. The threads were cut to the muzzle brake manufacturer's specification, the critical dimension is the thread depth, at .600" (+/- .010"). Operation complete, brake installed, customer happy.
Next time I talk with the customer I'm told he suffered a very mild baffle strike. Not good. Further information reveals that a rifle capable of sub 2 inch groups at 300 yards is now shooting 8" patterns at 100 yards. Not good. My suspicion is unstable bullet.
I get the rifle in the shop and confirm the threads are good, shoulder is good, muzzle crown is good and muzzle is flat. The rifle, without the brake, shoots a stabilized bullet and a test group of .750" at 50 yards (I have a very small test range). Install the muzzle brake and it confirms an unstable bullet. Looking at the carbon residue in the brake it appears there is about .300" of exposed thread and I noticed about a .700" total gap between the muzzle and first baffle in the brake. What was happening is the air being pushed out of the barrel by the bullet (precursor wave) was being disturbed by the exposed threads and other irregularities in that .700" gap, causing turbulence that destabilized the bullet.
My solution was to make a plug that threaded into the brake to take up that gap and eliminate the turbulence. After a bit of fitting I installed the brake and fired a test shot which revealed a stabilized bullet. A 50 yard group of .230" confirmed my solution.
In the future, any threads I cut for muzzle devices will be cut based off measurements from the device to ensure no irregularities exist to cause turbulent air. I hope that any practicing gunsmiths can benefit from this info.