M72 LAW launcher M190 35mm Sub-caliber adapter
Hopefully this is the right forum, its not an NFA device, and while not legally a firearm is somewhat related..... technically its an "emergency signaling device" according to the BATFE, and shoots a training rocket.
Amazing the sort of toys that you can play with when you grow up... :D
Here is FPSrussia shooting one
Anyway, its going to be a fun project