Quoted:
I was reading this link. http://www.engadget.com/2014/11/05/new-bullet-for-3d-printed-guns/

... And a couple thoughts just ran through my head....

First was "these would be more accurately called 'quick-change muzzle-loading barrels'".

Next was "that guy would be better off calling them ML barrels because then no federal law would apply.'"

Next thought was "if no fed law applies.... Machine gun!".

Here's the idea.... Each 3" rifled steel barrel would feed from a magazine into a barrel shroud.  Just make in so the barrel shroud is incapable of handling pressure and only as long as each "barrel".  Each barrel would be loaded and primed manually before being loaded into a magazine.  The bolt would then feed the barrel into the plastic barrel shroud.  Upon firing, the recoil/blowback would cycle the action.  There would be only one shot per barrel, which, IIRC, is the reason double barrel guns that fire both with one pull of the trigger are not classified as MGs by ATF.

Could I be on to something here?