None of the length is removed to install one, only the outside diameter and that depends on the thread size for the brake. If you want to maintain the same overall length with a brake then the barrel will need to be shortened, recrowned, and then threaded. Most brakes add from 1.5" to 2.5" to the overall length.
Depends on the caliber. Barrel wall thickness is what is important. IMO 1/2x28 is pretty thin on a .30 caliber. The bottom of the threads will run about .456 + or - a few .001 and the bore diameter is .308. That is only .148 difference or .074" per side. Not much steel left.
