Greetings,
I have been looking at several factory and aftermarket muzzle brakes (these are brakes where the outside diameter is the same or very close to that of the barrel) and they vary on how much bigger the brake inside diameter is vs. the caliber diameter.

How close to the barrel caliber diameter should the inside diameter of the brake be in order to be most effective?

I have been instructed by a learned Gunsmith who is also a mechanical engineer that the exit hole for the bullet should be 20 thou bigger than bullet diameter.

That is, 0.020" bigger than groove diameter of the rifle, for maximum efficiency of the brake.

Some factory muzzle brakes use the same diameter on everything from 6.5mm to .338-caliber.
I am not joking, but some manufacturers are.

Rip ...

Harrels Bros. has done extensive testing on this subject & according to them have found that .020 over bullet or groove diameter is the sweet spot for brake efficiency for their brakes.

Thanks everyone

I have read about the ".020" theory and believe it to be right.
I think that the bigger the OD of the brake, the better, and more ugly.
I am making one that has a big OD for effectiveness. Cannon style. ( very ugly!)

.020" is fine, IF the bore is concentric to the barrel's bore. Some are not.

We make our Muzzle Brake Reamers .020" over bullet/groove diameter.

Dave Manson

Assuming the length of the brake is short and locks up tight on a square shoulder on the barrel, 0.020 over groove diameter is fine. I’ve tried larger diameters to 0.050 with no discernible difference FWIW! Irregardless, it’s always worthwhile to check with a precision ground range rod or optical projector to make certain you have proper clearance.

.020 over bore diameter is what I standardized on several years ago. A couple of different manufacturers recommend .020 as well.

Jim
I have fitted muzzle brakes for a lot of Remingtons and some of their bores were as much as .017 out of concentric with the o.d of the barrel. Most of my brakes were machined to the same diameter as the barrel so what I did was dial indicate to the bore for threading and fitting of the id then I would dial indicate the o.d then turn down to .001 over o.d. and polish/blend with the barrel. When I was finished you could not find a seam where the brake met the barrel. It looked like it was an integral part of the barrel.

That is also the way I do it, it's Hard to imagine how some of those barrels will shoot