Oh well, I guess I better stock up on pre-WWI Austrian weaponry such as the 8mm Roth-Steyr and the 9mm Steyr. Semi-auto, stripper clip-loaded, permanent magazine. Along with a 16-shooter Henry rifle from 1860, I'll still have some real firepower. An 8-shot en bloc clip-fed rifle such as the M-1 Garand should also serve quite nicely; even a Sad Sack could deliver 24 rounds per minute, and a practiced, deft man could crank out upwards of 40-50 shots per minute. Of course, the plethora of 7-, 8-, and 9-round magazine-fed pistols could deliver upwards of 50 a minute, too.
Oh wait, that can't be right. Mass murder and mayhem can only be committed using "modern assault weapons," not century-old assault weapons. Not sure just why that is, but the