The Founders knew of multi-shot firearms!

and they came in fifty caliber
http://www.dave-cushman.net/shot/ducksfoot.html

Those who could afford it, could also own field guns:
http://www.americanrevolution.org/artillery.html

The founders also knew of rapid fire rifles
http://www.nps.gov/history/history/online_books/hh/22/hh22m.htm

Although after the Bill of Rights had been signed, President Thomas Jefferson had knowledge of arms that fired up to 22 rounds per minute!:

Air guns appear throughout other periods of history. The celebrated expedition headed by Lewis and Clark (1804) reportedly carried a .42" (10 mm) reservoir air gun, believed to be produced by Girandoni. It held 22 round balls in a tubular magazine mounted on the side of the barrel. The butt stock served as the air reservoir and had a working pressure of 800 PSI. The rifle was said to be capable of 22 aimed shots in one minute.
http://en.wikipedia.org/wiki/Air_rifle

Being men of vision, being able to forge a new country, they surely had the vision of technical advances (if not the know how) in all walks of life.