Really need more info. 45 ACP, 45 Colt, 454 Casull? 4 " Barrel? Whatcha got there?
Just a thought. If you are borrowing a pistol it's probably safe to assume your not real good with it yet, so therefore I wouldn't recomend hunting bear with it at all until you are more experienced. Just the fact alone that you didn't say it was a (such and such pistol in 45 so and so) kind of tells me your not real experienced with handguns in the first place.
I'm not trying to be abrasive or cut you down but 50 yard kill shots on a bear with a pistol and open sights requires a bit of skill. Hate to see a wounded bear get away or worse yet, attack somebody due to a poor shot.
Anyhow, thats my take. And again, I do not wish to offend at all so hopefully I didn't. Just your wording of the question makes me wonder how experienced you are. If your not, pack the 30-06.
I assume this is for hunting??