I agree albertan, the .45 Colt with up to 300gr. and .44 Mag. with up to 270gr., both in lever guns are preferable to me.

I've shot 2 black bears with a .45ACP - while working. The first was a head shot with a 230gr. FMJ handload at around factory speed - 840fps perhaps 20yards. The second was a rib shot at 35 yards with a 260gr. Speer HP, loaded with Speer's maximum load of W630 powder. The 5" Wilson barrel chronographed 960fps with that load and it killed the bear in seconds, stopping on the hide on the off side.
So- can you use a .45ACP to kill bear or deer - certainly. Would I do it again given the chance, probably not. I preferred my .44 Mag.marlin and .45 Colt M94 lever guns, both of which worked beautifully on bear and the .45 actually took two Elk for me with 300gr. HP cast, a sized down .45/90 bullet.