For Blackies, anything from the 6.5x55 on up will do, but if pinned down, I would choose my 30-06 with the 165 or 180 Partition. Very effective Bear medicine. Many years ago, I was involved in a bear cropping operation, due to orchard damage. I shot a lot of Black Bears over a 3 year period. Practically all were shot with the 30-06, and very few ever needed a second poke. I did shoot a few with a 300 Savage, and a half-dozen with the 6mm Remington, but didn't care for the latter, since it needed a second shot to settle a bigger bruin down permanently. The heavier rifles work well, but are unnecessary for a blackie. Regards, Eagleye