the founding fathers believed there was a line,
Really where was that line? Your understanding of privateers is completely flawed...so please tell us where that line was.
Odd that you lump full auto/select fire(federally regulated to near prohibition) and .50 cal into the same category. Heck If I had the money I could have walked out of the LGS with the .50cal on the wall today. Also while generally speaking calibers above .50 get classified as destructive devices, there are are examples (such as .600 nitro express) which are not strictly DDs. Furthermore the lesser(compared to full auto) regulation of DDs means that it would be relatively easy to get a stamp for a DD, versus purchasing one of the limited and dwindling number of full autos/select fires due to the registry being closed.
Where do I see the line? Somewhere around nuclear weapons. Practically speaking the destructive ability of someone who can afford a 747, or a fully equipped tank or warplane is pretty similar. The number of people who could afford these things are relatively few. With their resources if their intention were to cause destruction on a large scale, they would not need a tank or a warplane or any other weapons of war as you would put it. They could do so with legitimately purchased materials and/or materials purchased legally under false pretenses.