Well, I know that on the Lewis & Clark Expedition, a number of grizzly bears were killed with a .54-cal. round ball, so I suspect a .58 will kill them as well. In addition, a great number of Hawken rifles were made in calibers between .50 and .54 for Mountain Men who had to have something that would kill grizzlies, if necessary, and they also shot round balls!

However, unless you hit them someplace, like on the end of the nose while they are facing you so the bullet will go up the nasal cavity into the brain, or some comparable shot that gets into the brain, they will NOT usually drop dead on the spot! The bear will usually still be a live for some little time after the shot. It is during this period of time that the critter can cover a lot of ground, and do a lot of damage, to whatever happens to be in its path!

I have a .58 Hawken flintlock. It shoots the .570 PRB and the 570-grain Lyman 57730 flat-nose Minie bullet equally well, using 120 grains of FFg. The PRB is quite easy on your shoulder, but the Minie "lets you know something has happened" when it departs downrange!! I would not hesitate to use that rifle with the Minie and a powder charge of from 120 to 200 grains of FFgr for ANY ANIMAL on earth. But if I was after dangerous game in Alaska or Africa, I'd insist on having a backup guy wityh a .375 H&H or .458 Win. right there! Not because the gun is not adequate, but because it only shoots once, and who knows what can happen in the field!