Both are active to a degree dependent on food supply. At the height of the wolves killing elk in Yellowstone some big boar grizzlies stayed up feasting on wolf kills.For general knowledge, bears aren't true hibernators. They go into a slumber but can wake and kill as necessary. Especially brown/grizzly bears. Black bears are way less apt to come to life in kill mode. They go into a real slumber in their dens. Brown bear - not so much.
Where there is overlap the grizzlies are active earlier, and are prone to dig up black bears for an early snack.
Black bears also wander around in the winter, while cat hunting here, we cut as big a black bear track as I've seen, December 22, plenty of snow cover.
A dangerous situation exists when these bears don't have the required reserves to lay up for the winter. Other cases on file, but this one is from a respected member, that hopefully will respond as well.
I have been shooting a 338 lapua with 300gr matchking with great accuracy. What would be a good hunting bullet for long range brown bear? A matchking would work at long range but with a close encounter I don't think it would cut it. What would a good .375 bullet be for 500yards?