My idea is to forego the cartridge, and make a 5-shot electrically fired muzzleloader "pepperbox" similar in form to the
HK P11 for frogmen/SEAL-types.
Momentary switch for the trigger, (can be all printed too...)
http://www.amazon.com/Printer-1-75mm-Conductive-Filament-2-2lbs/dp/B00AF2L0W0 And the only non-printed bits would be a 9V battery, some small nichrome wire inserts, and some sort of simple IC that will activate/close circuit 1, 2, 3, 4, 5, in order. (still thinking about a ratchet, teeth, or other mech or something that can switch barrel firing contacts with all printable parts.) The safety is something that mechanically interrupts the circuits to the nichrome.
Mainly this design saves on the need for a breech, and you can maximize the strength of the polymer better with fewer parts, and done right, it could be a more "monolithic print job". Less assembly etc. And if/when printable bullets and propellant are possible, (extruded nitrocellulose, and high density polymer bullet?) you could even one day print new loaded barrel assemblies.
Maybe even get stacked charges "metalstorm-style" eventually.