Billy put one on one of his SR556's and it worked as advertised, though you want a front grip for better control of bursts and such.
Thing is the ATF will go after it in time, the only thing that lets it get past right now is there is nothing mechanical about the design. No springs like the 22 stock of old which the AFT nixed. It might take them a bit, but they will find a way to get that one also.
Any mechanism that allows multiple rounds to be fired with one pull of the trigger will be regulated eventually.