I think you will have to ask your guide if the 308 is enuf gun. I seriously don't think he will allow a 308 to hunt Browns, unless he is packing something very large and isn't scared to kill a wounded bear.

I would not personally trust an auto-loader on a hunt where there are dangerous game involved. A nice bolt action would be my choice with the minimum being 338, and preferably a 375.

With that being said if it was me and the shots were 150 yards or less I would probably get myself a Marlin guide gun in 45/70 with the stainless barrel/action.