You can get remarkably far with pretty crude tolerances, especially with lower pressure, pistol rounds. For example a direct-blowback, open-bolt submachine guns akin to what was used in WW2 can be pretty easily designed and built by anybody with a passable understanding of mechanics.

Again, we're talking about casting. Take a look at an Owen or a Sten and get back to me on what parts you think can be plausibly lost-PLA casted.