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.sadef

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 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/Metal_Storm http://en.wikipedia.org/wiki/Superposed_load 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.