After much searching I have found the maker of the door breacher muzzle on my 12C:
Its Fabarms... here is an example on one of their LE shotguns:
http://www.fabarm.com/law1.htm
I picked up this 12C out of Ft. Bragg about one year back. It came threaded for REM chokes, with ghost ring sights, and the attached door breacher. Sorry, I don't know anything about the breacher itself....