First thing is you need to check local regs. A lot of states do NOT allow any firearms while bow hunting.
If I had to carry one and was not "a good shot", get a snub nosed .454 Casull like the Ruger Super Redhawk Alaskan. It has a 2 1/2" barrel and not much good out past 25 yards anyway. Most anybody could hit a Griz sized animal at 25 yards!

Orrrrrrrrr . . . go with peper spray like http://www.udap.com/
If you did shoot a Griz, you'd almost have to show a missing arm or leg to prove to local authorities that you were truly being attacked.