A guy (retired German machinist) I know made 2 9MM's using part set MP44 pieces. Made his own 9MM blowback bolts and put Sterling SMG mags inside knock off MP44 mags.I shot both of them at the Creek several years ago. I did not think much of the idea until I shot them.
They actually worked and shot well. Same guy was making .22's from Airsoft but the availability of the factory made .22 Sturmgewehrs later killed his ability to sell the Airsoft reworks. Very probably the same guy is doing the Airsoft 9MM's.
He brought a FG42 semi he made to the Creek to show me a couple years ago. Made his own sheet metal rec. with his self made dies. Bought a few pieces from Estes Adams and made the rest himself. Neat FG, well done.