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.
Well, if he "can't" hit the vital spot, it doesn't much matter what he's carrying, now does it?
I've seen a bigbrown shot several times with a 378 Wby. that lived a couple weeks and was crow bait by then.
You guys make it sound like "if" you have a 375 H&H you have a dead bear, and if you have a 300 mag. you have a bear wounder???
The key to useing a 7 mag. or 300 mag. is to use "proper bullets"!!!! Many guides don't know a lot about bullets, so bigger is better in their mind. I owned and ran a gunshop in Alaska for many years, so i know something about what goes on there...
Did you know that a 7mm Rem. Mag. loaded with 175NP's will out penetrate 375 H&H loaded with 270 grainers???
To break a bear down properly, you need a bullet that expands well, yet still will penetrate deeeeeeeply!
Many guides like to see heart/lung shots because they figure they can and will stop a chatge if needed... They have no problem putting a second shot into the animial, and because you fired the first shot, your suposeto be happy...
Personally, i bust them down on the spot, and end the problem right there... I get close, and do the job right!
DM