“difficulty explaining why law-abiding citizens need a gun that can empty a 40-round magazine in a few seconds.”
Find me anyone who can manually empty a 40 round Mag-equipped AR in a “few seconds,” and dinner is on me! That would even be a trick with an M-16.
However, having that many without needing to reload might help if accosted by a gang.
That said, I wonder if the most visible chesty-beaters on FR might not actually be the ones to step forward if there was a real danger of that kind. Real heroes tend to be humble.