Here we see that with just a barrel, firing pin, feed, ejector, ammunition, and a 3d printer we can manufacture a lethal weapon.
Protip, if you have these parts and a whittling knife you also have a gun after a judicious camping trip.
"Oh no but designs exist for fully 3d printed guns that are really really shitty and worse than what you can make with a lathe and a mill". Can you 3d print a cartridge? That's the killy bit.