RE: 300 Ultra Mag for Brown Bear
No worries here. I would agree in that I think a 300 would be enough. Though, when I bought my last rifle, I chose the .338 LM just to be sure that I have all game covered.
The guide might be erring in the side of caution for whatever reason that might be. No doubt he has his reason. But if you think you can get it done with the .300 and you can hit that vital spot to drop a bear, then by all means take it with you. He is most likely just making his recomendations to give the average hunter a little something extra as a caution.