A gruesome holiday season exercise: Think of some firearms and accessories that might have added to the body counts of Aurora and Newtown. More starkly, imagine the means by which coming Auroras and Newtowns will be made more deadly. The exercise starts with a militarized baseline, as both shooters unloaded designed-for-damage rounds from high-capacity magazines loaded into assault rifles. Improving their killing efficiency would require one of two things: the ability to shoot more bullets faster, or more time. A fully automatic machine gun would provide the first. More minutes to hunt, meanwhile, might be gained by employing a noise...