I’m reminded of the book THE INDIAN WAR OF 1864 by Capt Eugene Ware in which he finds a bull bison and shoots it with his .44 Colt pistols. All the head shots bounced off. He used over thirty rounds of black powder paper cartridges to finally bring it down.
Bison have big heads. Their brain is only a small volume in it, and is protected by a lot of heavy bone.
I kinda doubt the “bounced off” report, but who knows the quality of powder he was using?