I use my .45 acp or .44 mag for bear protection at times (when I can't carry the 12 guage with 7+1 of the heaveyst and fastest slugs I can find), hard ball ammo should penitrate and it will be all about shot placement. Do I feel underarmed with the 45? yes and no. the 44 will hit a lot harder but I can get more acurate shots off with the 45 faster, either way it will be about shot placement (head) I feel far more confident that at the range that I would decided to shoot the 45 will put more lead on target.
I have drawn the .44 on a griz once, momma to one side of the trail cubs on the other me in the middle (very bad place to be but it was my fault) shot a round into the air and the cubs ran off and fortunatly momma bear joined them. I feel very strongly that if momma wanted to rip me a new one she would have been in the right. I still would have fired the gun empty but in a bear human contact it is 9 times out of 10 the humans fault.