Lage Manufacturing has been making conversion uppers for MAC-pattern machine pistols for a long time. They have a bunch of 9mm uppers in a variety of styles, some using magazines in the grip and some using forward-mounted Suomi drums. Well, a couple years ago Lage added a 5.56mm conversion to their lineup. This is an upper which is not regulated, and attaches to a registered full-auto MAC (or RPD, SWD, etc) lower assembly. It uses an AR-style bolt and gas system, standard AR magazines, and fires from an open bolt in fully automatic (the Lage uppers do not fit semiauto lowers, and they do not make a semiauto version).
Lage began with an AR-type recoil mechanism, using an AR spring and buffer in the buttstock of the conversion. This allowed a lot of user customization, but required drilling a hole in the back of the MAC frame to allow a rod from the bolt carrier to push on the buffer assembly. This was unpopular with many customers, and so Lage developed a second pattern, with a completely self-contained recoil mechanism. This IRM (Internal Recoil Mechanism) allows the use of folding stocks and doesn’t require any modification of the MAC frame, but it is a bit more complicated to assemble and disassemble.
Lage currently offers both recoil mechanisms, and has versions of the Max11 to fit both M11/9 and M11A1 machine guns. The M10 version is coming soon.