Muzzle Brake Exit Hole Diameter?

G

gzig5

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I am making a muzzle brake for a CZ rimfire rifle that has a threaded muzzle. What is the correct diameter of the exit hole at the front of the brake? I was thinking 10-15 thousandths over bullet diameter, but I don't have any experience owning/making a brake so I thought I would ask before I start making chips.
 
I believe 20 thou over is an accepted dimension, concentric to the bore.

Concentric to the bore is very important.
 
I use .020" over on the diameter. I would not use less. The holes can close up when improper (too soft) materials are used to make the brakes. I have witnessed this several times. Needless to say when the bullets start hitting the brake accuracy suffers.
 
I installed a Shrewd on a 7mm and debated on what to do because of the paint. I bored it out to .304 and stared at it for awhile wondering if the paint will screw it up after closing the tolerance. Maybe some paint will fly off and some will stick causing an uneven pattern. I was using Armacoat and baking it on, some said it will come off and some said theirs did not come off at all. I decided to ream it with an undersized 5/16" reamer which is .311 and thats .027" over bullet diameter. I used aluminum oxide to blast the heck out of it for the paint to stick well and painted with a baked on Armacoat. The paint has not come off after 200 rounds or so and a 308 bullet will not fit through the hole. FWIW, only the very end of the brake or the last baffle, about .200" thick, was reamed, by design the insides are larger in diameter already.

I'm not a smith by any means nor a machinist so ask a pro if you think what I did made any sense.
 
.012-.015 total will work fine as long as your not getting strikes. But light strikes with lead bullets can be very hard to see.
 
SWD,
I really have no idea what you are talking about with "light strikes" and how that affects the diameter of the hole. But, I have the info I need and thanks to all that posted.

Greg
 
If the bullet strikes the exit hole it is hard to see where a lead bullet hits vesus a jacketed bullet which leaves nice golden streaks. The perfect muzzle break has only enough clearance to keep the bullet from touching the exit hole. There is a noticable difference in performance between .010" total clearance and .020". But any ammount of bullet strike absoultly kills accuracy.
 
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