Seems kind of pointless to me, other than the Call of Duty wow factor for the EXTREME 14-18yo demographic.
If it's shell ejecting, that's a phony pony, especially if you're popping shells out several times a second.
If it isn't shell ejecting it'd just be another AEG, even if they somehow made it multi-barrel. Of course, if they made it multi-barrel it'd practically be a support weapon because every trigger pull = 3 BBs. You'd burn a ton of ammo.