It's me again, in this thread again. If you guessed that means something absurd, you're right! A few years ago, Ian posted a video about an early 20th-century French light
punt gun, a rather large shotgun meant for commercial hunting of waterfowl. At .920" caliber and with a 48" barrel, it's a big gun, but a (relatively) modest black powder load and a weight of 14 lbs, 4 oz means it can be shoulder-fired. A cartridge with 216 grains of black powder and 8 ounces of shot is what Ian mentions the gun using, which I converted to 68 pellets of FF (the largest waterfowling shot) and a very roughly guesstimated 40mm long case. As far as I know, there was no slug round, but you know some player's going to want one for the hell of it, so I included its stats.
Darne Canardiere Portatif (23.4x40mm black powder shotgun)
Ammo: 23.4x40mm shell
Wt: 6.46 kg
Mag: 1 round, loaded individually
Slug: ROF SS, Dam 9, Pen 2-4-6, Blk 11, Mag 1i, SS 7, Rng 30
Shot close: ROF SS, Dam 51, Pen Nil, Blk 11, Mag 1i, SS 7, Rng 30
Shot med: ROF 5x13/1x3, Dam 1, Pen Nil, Blk 11, Mag 1i, SS 7, Rng 30
This is actually not as absurd as the gun could be - with the smallest shot for waterfowl, T, it would have roughly 102-103 pellets per cartridge and still have enough energy for each to be Dam 1, so at close range it would be Dam 68 and at medium range the ROF would be 5x20/1x3.