Anybody who believes that the crafters of the US Constitution had no concept of rapid-fire weaponry is ignorant of firearm history. During the American Revolution, some obscure guy named GEORGE FUCKING WASHINGTON contracted with a gunmaker named Joseph Belton to buy 100 of his rifles employing a sliding hammer-and-frizzen device and the principle of superimposed charges that could discharge as many as 16 rounds as fast as you could reposition the action and pull the trigger.
There are no known surviving Belton rifles but this is a photo of an early 19th Century 12-shot adaptation of the Belton design made by Isaiah Jennings. Which was built commercially in a variety of configurations, some of which were sold to the New York state militia.
But General Washington backed out of the deal because he thought Belton was asking for "unreasonable compensation."
But wait, there's more!
Thomas Jefferson owned a 20-shot Girandoni rifle, which could fire all 20 rounds in under 30 seconds. He liked it so well that when he outfitted the Lewis & Clark expedition, he saw to it that they had two of them, which became their primary meat guns.
But wait, there's more!
In 1717, James Puckle invented what essentially was a 1.25-caliber, tripod-mounted, hand-cranked 'revolver' he hoped to market to the British navy to be used in repelling anyone who would board their ships by force. It was similar in function to Richard Gatling's revolving "machine" gun, which it predated by about 140 years.
The Puckle gun didn't work well but its lasting contribution to the evolution of the species was the term Puckle used to desctibe his invention in the sales brochure. He called it a "machine gun."
But wait, there's more!
In 1680, Michele Lorenzoni invented a lever-action flintlock that loaded ball and powder from separate internal magzines when the shooter cycled a lever underneath the action, and could be reloaded as fast as the shooter could cycle the lever.
A few years later an Englishman named John Cookson started manufacturing Lorenzoni rifles in London. Some of his guns from as early as 1690 still exist. By 1750 (and if your math is weak, 1750 comes before 1787), John Shaw (another Englishman) had moved to Boston -- the hotbed of the American Revolution -- and was manufacturing Lorenzoni-pattern rifles under the name "Shaw's Cookson Volitional Repeater."
And remind me how many of the Founding Fathers were living in Boston in 1750?
But wait, there's more!
Nobody knows for sure who designed it but some time in the early 17th Century (possibly as early as 1630), the Kalthoff iron works in Denmark was manufacturing a lever-actuated repeating rifle that was similar in its operation to the Lorenzoni.
Above, a surviving Kalthoff repeating flintlock rifle.
Below, an animation of how cycling the Kalthoff's operating lever loaded powder and shot from separate internal magazines. Yes, it had to be aimed upwards to facilitate its gravity-assisted operation.
But wait, there's more!
This is a German rifle whose origins are lost to history but it dates from about 1580. What's unique about it is that it fired two sequences of superimposed charges in rapid succession by means of a gunpowder-filled "touch hole" running the length of each of the bullets, which is exactly the same technology as is used in a Roman candle. And in the modern lexicon, any firearm that discharges more than one shot from a single operation of the trigger is called a "machine gun."
Behold, the 1580 machine gun:
But wait, there's more!
The image below is from a late 14th Century Chinese military historical work called the Huolongjing. It depicts a weapon it called the "divine fire arrow shield," which basically was a box full of big-ass bottle rockets with arrowheads on them, which could be fired either sequentially or in a single volley.
And this wasn't a depiction of a contemporary weapon, this was something that already was centuries old when this was printed. Which would put it back to within a century or two of the invention of gunpowder.
Except for the Girandoni, all of these were too fickle in operation or too expensive/difficult to build to be practical, but their impracticality isn't the point. The point is that all the way back to within a couple of hundred years of the discovery of gunpowder, history records that they weren't satisfied with single shot firearms. Not even from the very beginning. They wanted multi-shot arms, and the faster they fired, the better.
The problem was, the supporting technology necessary to make something like the AR-15 hadn't been developed yet. It wasn't that such a weapon was inconceivable, it wasn't lack of imagination, it was the primitive nature of science -- primarily chemistry and metallurgy -- that was impeding its development.
And in the case of the firearm, the linchpin was the self-contained, one-piece metallic cartridge. Except there was never going to be a practical metallic cartridge until someone invented the percussion ignition device (AKA the "percussion cap" or "primer"). But once those two bits of technology came to pass, the rest already was in place and it was off to the races.
If that's too much to get your head around, I can make the same analogy with aviation.
Man probably has dreamt of flying since the first time he looked up in the sky and saw birds on the wing. The Greek myth of Daedalus and Icarus and their wax wings probably dates from about the middle of the second millennium BC.
Da Vinci sketched conceptual helicopters and ornithopters in the 15th Century.
The Montgolfier brothers started sending farm animals aloft in their hot air balloons in the early 1780s. By 1783, using a more advanced prototype, Étienne Montgolfier became the first human to experience sustained flight.
In 1871, Alphonse Pénaud held a public demonstration of his latest invention, a self-propelled (unmanned) airplane that was the predecessor to the rubber-band powered toy airplane. It remained aloft for 11 seconds.
In 1891, Otto Lillienthal designed, built and flew what in modern terms would be called "a hang glider." Lillienthal was the world's first true aviator.
And in 1903, the Wright Brothers achieved man's 3500-year-old dream of sustained flight.
But aviation wasn't done there. Da Vinci conceived of and designed a crude helicopter 400 years earlier, but no one managed to build a practical 'copter until the late 1930s. Which might seem odd because the helicopter's method for generating lift is much more straightforward and intuitive than the principle that allows an airplane's wing work. Especially since the invention of the electric fan, everyone in the modern world could understand (at least a simplified explanation of) how a helicopter could achieve flight. So how come the airplane got there first?
Intuitive or no, the helicopter's means of achieving lift is far less efficient than the airplane's wing, which means the helicopter is more power-hungry. In time it was found that a helicopter needed an engine that could produce at least one horsepower from no more than two pounds of weight. The Wright brothers' first airplane's engine made just eight horsepower but weighed 200 lbs. So the linchpin in the advent of the practical helicopter was the development of an internal combustion engine that could produce sufficient horsepower to its weight. The airplane got there first because its more efficient method of generating lift let it get by with a less sopisticated engine.
And so it was with firearms. They knew from the start what they wanted, they just couldn't get there yet because they had to wait on the supporting technologies. The rest was a matter of incremental steps. Ah, but a man's reach should exceed his grasp, Or what's a heaven for?
The only reason George Washington ever got involved with that Belton character and his superimposed charge rifles in the first place was because Washington's friend, a certain Dr. Benjamin Franklin, had written to him stating that he had seen Belton's guns and was impressed with their potential. And he suggested that Washington might benefit by acquainting himself with Belton's work. So it's not like it wasn't already common knowledge.
And anyone who claims that the founding fathers could not have conceived of anything like a rapid firing repeating firearm is at the very least ignorant of history.
And just because you've got a bug up your ass about the AR-15 doesn't qualify you to dictate firearms law to the law-abiding.