Some other replies:
1. The Second Amendment, when conceived, was, in effect, guaranteeing the right of the people to own the most potent military weapons of the time--certainly weapons on par with those in the armories of the best-armed nations of the time. In fact, it was not uncommon for (wealthy) private citizens to own cannons or even warships.
2. The Second Amendment is carefully worded in such a way as to make it clear that its purpose is to insure that the armed citizens could help insure the "security of a free state". Clearly, the authors intended to protect the right of the citizens to arm themselves to the point of being an effective military fighting force.
3. Multi-shot (multi-barrel) firearms were not unheard of during the timeframe that the constitution was penned. There were also breechloaders (such as the Ferguson Rifle) which could enable the user to fire at a rate of perhaps 2 to 3 times greater than the single-shot muzzle-loading muskets. Yet the constitution makes no attempt to exclude such weapons from private hands.