Rifling of gun barrels. If the Heller definition had been in place then, they would have been prohibited. Are you suggesting that the firearms industry have all disbanded their R&D departments and that no new technologies will ever be forthcoming?
and I lived in Maine for a quarter of a century, and knew HUNDREDS of folks who hunted. I could count the number of muzzle loader owners in that group on one hand, even if I'd lost three fingers. Clearly, your definition of the word "common" and mine are different. We'll have to agree to disagree on that issue.
I have to say your "rifling of gun barrels" was a pretty slick answer and more thought out than about 90% of anti-gun retorts. that said however, I am not certain that taking an existing smooth-bore arm and rifling the barrel to improve accuracy reduces the commonality.
The point is that "arms" today bear little to no resemblance to those the founders knew. At each advancement, arms become more and more effective, more efficient, and more lethal. I happen to believe that government has a role to play in regulating which of those new technologies is solely for military use, and which ones ought to be available to private citizens. And if they do so after the use of some new technology has, perhaps, become somewhat common, I am fine with that, too.