From what I've read about handguns used against animals, .45 ACP is usually a poor choice on bears. As is 9mm, 40S&W, and other common service pistol calibers. They're made for stopping people (or people that act like animals), not animals themselves. Many have been killed or seriously injured underestimating what a certain firearm will do against a large animal.

In fact, one article I read even said that, in the writer's experience as a bear hunter living in a big bear area, heavy .357 magnum loads perform noticeably better against bears than .45acp. The heavy .357 magnum loads can penetrate to places where .45 acp simply can't. However, it's not an ideal bear caliber in their opinion, it's just better than .45acp (which, a lot of people seem to think is an acceptable bear caliber).

With that being the case, I've formed the opinion that, at an absolute minimum for an outdoor handgun I would want heavy 10mm or heavy .357 magnum loads in a long barreled gun (5"+). I would not, however, feel totally comfortable with that. I would only be carrying that in places where the risk of big animals coming after me is very low.

My preferred handgun choice for a place where bears and such are common would be a .44 magnum or .454 casull. I've read of multiple documented cases where they have worked to stop large bears and other large dangerous animals on the charge. Most recently, that link mentioned above where a short barreled .454 casull was used to stop a bear in Alaska. Still, I wouldn't be 100% comfortable, but I would probably be about as happy as I'm gonna get with only a handgun.

With all that in mind, I still wouldn't want to be using a handgun against a bear or any other big dangerous critter if my life, or the life of another, depended on it. I'd much rather have something like a 12 gauge shotgun firing magnum loads, a .45/70 lever gun, or a medium to big bore rifle. The only reason I would consider a handgun is simply because carrying a long gun around most of the time just isn't practical (nor is it usually quick to get into action if it's strapped to a pack or laying against a tree).