Not sure on the 12ga slug for buffalo. To drop it humanely, you need a decent hunting round.
I forgot to mention this morning - the buffalo cow I dropped was in running the rather large corral. I was using a lever-action Marlin .30-30 with soft-nose. Shades of the old days.
Took a couple of shots, apparently she was ducking when I fired. Actually, she was stopping at the back of a building and turning around, and since she was stopped, I shot - as she started running, so I had 3 close holes in her hump - right above the spine I was going for. I finally figured out what was happening and dropped her. Bled her out right there - the other 2 were already gone - and took her to the fellow helping me to butcher them.
I had kept them in the corral to keep them from jumping anymore fences, fed them corn and hay. They dressed out at 190 a side. Five years later I still have some in the freezer, absolutely fantastic meat.
It's good to be a carnivore.
And the St. Jo cops ruined good buffalo, but then it wasn't their property.