Per the USC: https://www.law.cornell.edu/uscode/text/26/5845 Note that it says one shot, not one round, or one piece of ammunition, but one shot. If your side/side or over/under simultaneously fires both barrels with one trigger pull, is is a machinegun?
Presuming that the trigger can be manipulated to fire sequentially or simultaneously by how far the trigger is pulled, can the the simultaneous firing mode be described as separate functions of the trigger?
Efficacy as a semi-automatic double. Both barrels at the same time woks better than one then the other.
To answer your question: Combi weapons are ok BUT they must be combi weapons IE two bolts two barrels TWO TRIGGERS etc. They've got some already: http://www.thetruthaboutguns.com/20...review-arsenal-af2011-a1-double-barrel-45acp/ http://www.gunsandammo.com/video/first-look-gilboa-snake-double-barrel-ar-15/ Methinks that the reason you see doubles like that instead of how you suggest is to get around the definition. Shot would seem to refer to a single round in this instance. So one trigger pull for two rounds, not cool, but TWO side by side triggers pulled simultaneously is honky dorry.
I don't know of any that do that. Do you? Two triggers, two barrels. Rare that 1 trigger shoots sequentially. I don't know of any that 1 trigger fires both barrels at the same instant OTHER than some antique percussion or flintlock handguns with multiple barrels - and they are exempt as pre-1898.
A gun trigger assembly could be built so that a pull of the trigger did 3 functions sequentially as it was pulled back on 1 pull for basically a 3 round burst, though they probably challenge that and then just rewrite the rule. A gatlin gun setup - a MANUAL rotary with cams against a trigger - currently legal. However, it can not be electric or spring loaded.