Fully automatic weapons were unknown in the founders' time but it's possible (if difficult) for private individuals to own them. Whereas private individuals can no longer own even historically-accurate warships armed with cannons. There are all sorts of restriction that intrude on the natural interpretation of 2A (at least if we ignore the "being" clause, which the courts don't seem to interpret in the natural way at all), but I don't see a particular correlation with when a weapon was invented.