The 2nd amendment was written right after the time when the US Government (such as it was) was borrowing privately owned cannon for the war effort.
I don't know about the Post, but a 6 pounder with grape seems like it's a lot more dangerous, and the founders were hunky-dorey with civilians owning that combo.
That same 6 pounder, grape shot, and powder are still legal to own today, under federal law.
/johnny
You can also purchase & own a live-fire black powder Gatling gun, in 36, 45 or 50 calibers.