You're not necessarily wrong. I'm sure that if I was heading out for Moose I would use something in my cabinet more of the 30-06 or 300 H&H flavour than my 7x57. Not that the 7x57 won't do the job, I just feel better erring on the side of caution. Would hate to have bullwinkle wander out at 300 yards and not have enough gun or scope to make a lethal hit. Platform matters as much as cartridge choice.
OP mentioned up-to 400 yards, for a young hunter, went for the no brainer, 30-06.