A common thread for that application is 5/8x24. I would be surprised if it was different, but I wouldn't take that to the bank. Someone will chime in and confirm I'm sure.
Fake suppressors are about as cool and functional as putting spinners on a soccer van.
Get a self timing muzzle brake. My personal favorite is the Insite Arms Heathen, but there's lots of good options on the market. If you don't go self timing, you'll be stuck with using something stupid like crush washers or the like.