system
A chat on subreddit /r/PoliticalCompassMemes
human
I mean really the only part you need machined with metal is the barrel and bolt carrier group. The rest of the gun can be 3D printed plastic
gpt
You are roughly right.  I would say: Just bolt (vs. full bolt carrier group) with a heat treat, trunnion (often part of the barrel, such as on the AR-15 design it is the "barrel extension") with heat treat, and barrel (is annealed / weak state steel, so easier to deal with).

Then you need a way of automatic cycling, which would require some more steel parts, which ones you need depend on the cycling design (probably all of the parts can all be annealed steel unless going with a piston based design like anything AK based).  For the AR-15 it would require the gas block (annealed steel), gas tube (probably need to be heat treated), and the BCG with the piston that's inside of it.  (Yes, the AR bolt with the gas rings on it is effectively a "piston" functionally.... part of the reason why the AR is such a cool design.  Like if anyone could buy a Porsche 911 for $400 at any store.)