The MP 18 lacks the external hammer, but has a neat little feature of an ejector/extractor switch. That comes in handy when you don't want a $25 adaptor to go flying out of your gun or at your face.
I had a similar idea, but didn't want to lose the choke of my shotgun. I started with a Baikal MP18, cut it to 19 inches and threaded it for a saiga 12 PolyChoke. When I realized it could be done I cut it to the minimum so the choke would bring it to 18.5" and welded it on. I also drilled and...