"AK-47" is typically 7.62 mm x 39 mm
.308 is 7.62 x 51
Mosin is 7.62 x 54
.30-06 is 7.62 x 63
.300 Winchester Magnum is 7.62 x 67
If you're going after bear, I recommend carrying a .300 Win Mag or a .30-06, preferably magazine fed (M1/M14), and a large handgun (.45ACP, .44 Magnum, .357 Magnum). .50AEs and .500 S&W can be difficult to handle, not to mention magazine capacity is small.
There's a video on Youtube of a Redneck gone pig hunting. He stumbles onto a 350 lb pig and puts one .50AE into the animal's rump. Enraged, it turns around. He fires five more rounds. The pig tries to get up and come after him. He fires one last round into a place he's told to hit by his guide. The pig drops dead. He was firing a Desert Eagle .50AE, which only holds seven rounds. He was out of ammunition and less than ten feet from the pig.
Don't make that mistake or it could be your life. And don't try to do a head shot on a deer.