Actually, I believe Baby was a muzzleloader. Among the BP cartridges, the .577 BP Express, 3 1/4-inch was one of the biggest rifle cartridges, and they you go to the "Gauge rifles" of 4- 8- 10- and 12-bore. I have never seen case specifications or factory load data for any of these, and would be curious to know the details, although in their heyday, the chaps who could afford them could probably afford the virtually hand-made ammo that went with a particular gun.