At the time the amendment was crafted, they allowed the most modern weapons available at that time. Also, it should probably be noted, the same weapons used by military forces at that time, a.k.a. weapons of war. Also, while there weren't fully functional automatic weapons at the time, they did have weapons such as the Puckle Gun, patented in 1718, and is a mounted, portable, crew-served revolving machine gun. Do you really think they believe they'd reached the epitome of firearms technology? Also, it's worth noting, cannons are still completely legal to own, and you don't even need a license if firing shot or grape shot type of shell. Explosive rounds require a license for each round.
Why aren't motorways governed by laws applicable to the horse and buggy if we're to hold strictly to what the founding fathers could envision? Why were all those slaves emancipated? Why are women allowed to vote now?
In other words, what Macs said.