Tactical Innovations will soon be selling a .22 LR upper receiver for the M11/9 submachine gun (semi-auto clones are not compatible). The receiver uses American 180 .22 drum magazines which can hold up to 275 rounds.
The AM15 Upper Receiver system, along with quad rail, a 165 rd drum and winder will cost $1395 plus shipping. They will be shipping it as soon as they get ATF approval.
The below youtube video from 2007, posted by a company called Thompson Machine, shows an upper receiver for the M11/9 that uses the American 180 .22 drums …
[ Many thanks to Jeff for emailing me the link. ]