So just an update from the guy who hijacked the thread to cry about his own muzzle brake woes:
My split rethreading die finally showed up after being accidentally diverted to the wrong state.
When I first put it on the muzzle threads, it couldn't even close all the way. The major diameter of the threads was out of spec enough, which explains why some of my muzzle devices wouldn't even thread on completely.
So after several runs, a liberal amount of cutting oil, and way more material removed than I expected, and now everything spins on without any problem.
Thanks for all the info for everyone who chimed in with suggestions on how to fix this!