They do have subsonic conversions, .300 Blackout is particularly popular for the Tacticool set, but the Stoner action is pretty loud on it's own. (I'm not going into the harmonics of that buffer, and the lack of any other sound deadening in that aluminum chassis, unless someone wants to read it.)
So, depending on what you want the Muffler for, .223 is quiet enough supersonically to fire it with a can, instead of hearing protection, outdoors. (The muzzle break also makes a lot of noise around the muzzle, which is dangerous for an unprotected shooter.)
If you want to play Commando, then the action cycling is a hell of a lot quieter than the Muzzle break, however the non-directional shockwave of the supersonic bullet can distract people (Depending on the cover around) from that sound as well, so it's nigh impossible to trace the shot back to the source, with the echos.
Toe was nice enough to explain the pun, so nobody gets it, and gets a point for saving me the trouble.