1/2x28 for this barrel.
It is incorrect, the barrel is threaded 1/2-36. This is the second "expert answer" on the list and I purchased a very nice muzzle brake based on this answer. Didn't fit, fortunately I didn't destroy the threads on the barrel by putting a wrench on it before figuring out the problem.
So, after buying a cool muzzle brake in 1/2-28 that doesn't fit I did a bit of research. It appears that these are threaded in 1/2-36 NOT 1/2-28.