Thomas Jefferson owned a .45 caliber 22-shot semiautomatic rifle. Two of them, actually; gave them to Lewis & Clark who used them extensively to discourage potential attackers.
The Puckle Gun, an early fully automatic (crank driven) rifle, was demonstrated to leaders of the Revolution. Not used only because production was limited.
The authors of the RKBA would have seen the M16 as a refinement of what they were familiar with, not as some unthinkable amazement not intended for the 2nd.