Buy a 30-06, load it with 200-220 gr Nosler partitions or a 180 gr Barnes TTSX.
These are recomendations from the Alaska game and fish dept on bear defense. According to their tests the only rounds that outperformed the heavy loaded 30-06 were 375 mags. and larger. Several others have duplicated these tests and came to the same conclusion.
The 300 mags tied the 30-06 in performance at closer ranges and only offered an advantage at long range. The 30-06 out penetrated everything between .30-.375, including 338 mag, 45-70, 444, and 35 Whelen.