There are not standards. The guides/outfitters make their own rules based on there own experiences/observations.
Use what you are comfortable with that is within their guidelines... Personally, I would use a nice lever-action 45-70...
"Laws that forbid the carrying of arms...disarm only those who are neither inclined nor determined to commit crimes...Such laws make things worse for the assaulted and better for the assailants; they serve rather to encourage than to prevent homicides, for an unarmed man may be attacked with greater confidence than an armed man." - Thomas Jefferson, 1776