Let freedom ring.
http://defensedistributed.com/Can you imagine the problems this creates for tyrrany?
Let's say you're a tin-pot despot. Pick a country, doesn't matter.
You're a tyrant and you prohibit ownership of any firearms. And you tax at 90% or higher, and you have a caste system, and you have a maniacal state religion, and you have goon squads running around with machetes.
SOMEONE gets their hands on a RepRap. They print out more RepRap printers, then they print out a bunch of single-shot firearms.
You could probably build a 100% plastic rimfire firearm, including barrel. It only needs to last long enough to kill one soldier or goon that is better armed, once. But a plastic rimfire semiauto would probably last dozens of rounds reliably, maybe hundreds.
Heck, make the barrel thick enough, or make a plastic insert to put inside a 3/4 inch or 1 inch steel pipe... and you can shoot 9mm, .223, 7.62x39, whatever from it. At least once.
This is freedom and liberation for the minority African tribes that routinely get savaged by thugs.
This is a means to revolution for Falun Gong in western China.
This is something that EVERY criminal with a gang behind him, needs to worry about, in places where potential victims are now currently guaranteed to be unarmed.