I understand that guns aren't hard to make - never mind 3D printing, any machinist can make a submachine gun. [1] But what about ammunition?
http://www.scribd.com/doc/2624298/sten-mk2-complete-machine-...
http://www.scribd.com/doc/2624298/sten-mk2-complete-machine-...
Not terribly difficult considering you can reuse the casings. The primer is by far the most difficult part, but not impossible. Propellant is no more difficult to make than meth.