Some of the Hollywood blank guns are propane fired, the outer shell looks like a real gun but inside it's just a combustion chamber and a spark igniter. Flame shoots out the muzzle and looks/sounds like an MG. If you look real careful at Warren Beatty shooting the Tommy gun in Dick Tracy you can see the propane hose laying on the ground and running up his pant leg to feed the gun. Something like that would easily pass muster as long as the "receiver" couldn't accept cartridge firing parts.
Friend.....you have not lived in a Free Country for a very long time. Get used to it, cause it ain't likely to change in your life time.
When idiots tell you that you live in a Free Country, feel "free" to laugh at them.