[rdl]

While barrels benefit from quality (and even things like titanium firing pins, $200 special wooden grips, $3000 optics, etc.), I am confident I could show a minimally literate 8th grader how to make a serviceable open-bolt fully automatic weapon (say a 9mm SMG I guess) with a random high school machine shop in 3 days, capable of firing <4 MOA, >900fps muzzle velocity, >95% reliability with Wolf 9mm ammunition for 500 rounds.

Basically the same thing was done in WW2.