>>64373035
If the ammo can't be made as required the gun can be made to work around the ammo. Hulkhogan made a printable action that will fire four chambers semi auto, even if your ammo has a one in four fire rate you can get a shot off. People have been iterating on it in some cool ways.

>>64322276
Bambus are good and easy, a1 for pla+, one of the enclosed ones if you want to do nylon.

>>64323643
Legal process to make a machine gun? You need to file for the correct federal firearms license and pay the special occupation tax, it's involved and expensive but doable.

>>64078920
Bejust or bedead is almost done with a miniaturized version of the four banger.