i am no master machinist, but to turn a 5/8" thread the barrel gets turned down to .625.
yes, i threaded a barrel that measured .498, a bushy carbon 15 with the quick detach muzzle brake. the barrel was thin, but then stepped up.
a shoulder to seat against does not need to be large. most barrels have some taper, while more might seem like better, i think anything over .030 would work. or, i have not tried it yet, but a jamb nut. also, when i cut threads, i do not cut to spec. i cut until it fits MY muzzle device nicely. most of my commercial threads are quite loose, the ones i thread fit much better.