The Oly I had back in 1989 had 2 fat pins crossed through the threaded area like similar to that as seen on the front sight base... just grind till you find them and they fall out, providing yours is done in similar fashion as mine was.
Take and grind away at the first 1/2-3/4" of the flash higer until it comes loose. If you try to twist it, you'll screw up the end of the barrel.
Bushmaster takes a different approach, does the job, they drill hole in muzzle device,, then into threaded area about 1 pin diameter, shove pin in to lock it, weld over top... never did see the pins, just the 2 small holes in the thread area.