This argument is based on ignorance of the history of firearms. It is true that in 1791 the most common firearms were handguns or long guns that had to be reloaded after every shot. But it is not true that repeating arms, which can fire multiple times without reloading, were unimagined in 1791. To the contrary, repeating arms long predate the 1606 founding of the first English colony in America. As of 1791, repeating arms were available but expensive.
This article explains why the price of repeating arms declined so steeply. Then it describes some of the repeating arms that were already in use when the Second Amendment was ratified, including the 22-shot rifle that was later carried on the Lewis and Clark expedition.
One of the men to credit for why repeating arms became much less expensive during the 19th century is James Madison, author of the Second Amendment. During Madison's presidency (1809-17), Secretary of War James Monroe (who would succeed Madison as president), successfully promoted legislation to foster the development of firearms technology. In particular, the federal armories at Springfield, Mass., and Harpers Ferry, Va., were ordered to invent the means of producing firearms with interchangeable parts.