To crudely rifle a barrel you just need a press. Or you can make a machine like this: https://www.alloutdoor.com/2017/04/17/watch-homemade-barrel-.... I once saw a video of a machine which did not require any machine tools, you'd just make a guide and then manually cut the riflings. All you need there is a lot of patience.
For these designs you also do not need 3d printing at all. You just need suitable pipes, or sheets of metal that you bend and either weld or use fasteners to make the receiver. You are right though, designing and building reliable magazines is probably going to be one of the things that require a lot of attention to details.
Depending on the magazine dimensions (single stack is hard) Reliable magazines can be largely 3d printed, as can the follower and the jigs used to bend piano wire into proper springs. The 3d.printed.mag will usually have less capacity than a metal one though.
For these designs you also do not need 3d printing at all. You just need suitable pipes, or sheets of metal that you bend and either weld or use fasteners to make the receiver. You are right though, designing and building reliable magazines is probably going to be one of the things that require a lot of attention to details.
There are books that have full blueprints for crude SMGs, here is an example: https://www.amazon.com/Do-yourself-Submachine-Gun-Lightweigh...