All brakes need .020 minimum clearance to work correctly.
A .308 needs to have a .328 bore that is concentric with the bore. (.010 thousandths clearance on
any side of the bullet) To prevent bullet strikes.
The .338 bore needs to be .358.
You were wise to check the bullet in the brake. I once bought a brake that had the right bore diameter in all but the port next to the crown. (Apparently the reamer did not go far enough
in the brake and did not open up the inside hole).
That hole was .250 (Probably the pilot hole) and the rifle I was going to use it on was a .338.
I always check all holes by dropping a bullet through the brake to make sure. This was scary
and I immediately called the maker and informed him that this had happened, to prevent this
from happening to someone else.
I have heard of muzzle brakes
being shot off the rifle and suspect this one of the reasons.
J E CUSTOM