[Wogef]

If you are going for the "shell as a barrel" solution you might as well maximize the utility and make it either silenced (captive piston) ammo: http://www.sadefensejournal.com/wp/?p=1812

Or make it electronically fired multishot using a superposed load like the Metalstorm: http://en.wikipedia.org/wiki/Metal_Storm http://en.wikipedia.org/wiki/Superposed_load

That being said, while metal home use printers won't be feasible anytime soon, FDM plastics are getting much, much stronger (Ulthem etc) and being released to prosumer market at an impressive rate. Having a barrel stand up to a dozen shots or so will not be a problem for long.