At the time of The Revolution and many years after, private individuals owned artillery, and warships (the tanks of their day). There is a reason the founders used the word "arms" not "guns".
Sure, and at the time, those arms were the most technologically advanced and deadly weapons available around the world. That was the frame of mind they were in when they wrote the Constitution, that people having these super machines of destruction was the best way to keep the government free from tyranny.
Nowadays, one person with a bad outlook on life and a fully armed F18 could kill literally tens of thousands of people in a few seconds. Its a different world than it was in the 18th century, you can't deny that.