First off, they would have said for muskets only, but they didn't. Thomas Jefferson was even sent a letter asking if a civilian could have a FUCKING cannon on their ship and Jefferson replied, of course. That's your right to do so. Secondly, the Constitutional Congress actually were well aware of the existence of automatic weapons and even commissioned the Puckle Gun for use in the militias, but wound up being too expensive. There were several guns of their kind by 1781 and the US had no problem with their citizens owning any of them. The point of the article in the bill was that the militia would be equally equipped as the army was. Washington even had to fight US militias that rose up all the time over the next few decades. The easiest thing in the world for Washington to do was revoke the second, but they believed in it.