The Founders knew of multi-shot firearms!
and they came in fifty caliber
Those who could afford it, could also own field guns:
The founders also knew of rapid fire rifles
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.
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.