Things like this have the potential to make the whole gun control debate moot:
http://www.forbes.com/sites/andygree..._medium=social
Key quote:
"All sixteen pieces of the Liberator prototype were printed in ABS plastic with a Dimension SST printer from 3D printing company Stratasys, with the exception of a single nail that’s used as a firing pin. The gun is designed to fire standard handgun rounds, using interchangeable barrels for different calibers of ammunition."
It is not clear if the interchangeable barrels are also printed or if they are separate metal pieces that must be added.