Ok folks, but especially you Jenni and VNV.
You want a high rate of fire? You want something that wasn't a powder burner?
TA DA!
I present, for your consideration, the Girardoni repeating air rifle.
Girardoni air rifle - Wikipedia
A 20 shot magazine, available in .46 and .51 calibers and with a range of 125 yards on a single pumped up charge of the air reservoir. No bayonet lug, so it wouldn't be considered an assault weapon. And yet, the Founders didn't bother to put a codicil to the 2nd Amendment to moderate the horror of this relentless killer.