(I once spent a couple of days with the guys from Florida Fish and Wildlife trying to figure out if it was legal to hunt monkeys in the state of Florida, and so I know. The hardest part is getting people to understand that no, you're serious! You want to know!)
This is one of those things that I honestly don't know whether it'd be illegal or not, because the basic activity is perfectly lawful. I'm just not sure (and neither is the author, though he operates on the assumption that the game warden would be likely to haul you in anyway, on general principles) whether it's lawful to do it quite this way. On the other hand, THIS IS EVERYTHING I LOVE ABOUT MEN: a guy builds his own Civil-War-type 12 pound mountain howitzer and goes deer hunting with it. Obviously, a 12-pound cannonball would be a little excessive (and hard to aim accurately), so he uses canister shot.
That would be a hundred and forty-eight .69 caliber lead musket balls, propelled by half a pound of powder.
Advises the author: Also be sure the deer is no closer than 100 yards. A minimum distance of 100 yards is a "must" for reasonable meat recovery. ( less damage ) You don't want to hit the deer with all 148 musket balls.
I got nothin' to add, folks. (And yes, he ate the deer. The field-dressed carcass (photo at link) must be seen to be believed.)