I drilled and pinned a muzzle brake on a Mini-14, and that barrel steel was very tough. We ended up using a carbide end mill in the Bridgeport. If you get the die started the least bit crooked, it's going to be obvious. If it were me, I'd skip the muzzle brake and send it to D-Tech for one of his compensators, too. It will be shorter, quieter, more effective and come out right.
Good luck if you try it yourself. Make sure you're aligned before you start cutting threads, use cutting oil, and have everything locked down in a vise, because I'll bet ti's going to take some major torque. /ubbthreads/images/graemlins/wink.gif