Bear rifles are bolt-actions. Nothing else is as reliable or can handle the powerful cartridges that you need. The best cartridges are, starting at the low end of the power scale, the .338 Winchester Magnum, .340 Weatherby Magnum, .338 Remington Ultra Magnum, and .375 H&H. Of these, the .338 kicks the least and may be the best choice if bears are not your primary game. The .340 and .338 RUM also use .338 bullets, but at much higher velocity, and have a lot more recoil. The .375 H&H shoots even heavier bullets and is a huge favorite among Alaskan guides because it’s a stopper.