I think on big game the maximum distance to use iron sights is the distance under field conditions (NOT from a bench rest !) is that a hunter can hit a 9" kill zone consistently from standing or sitting ;the positions used most in the field. In the Marine Corps we qualify every year with a service rifle out to 500 yards . Most Marines can hit a man at 500 almost every time with a service rifle with issued iron sights . That being said a target is not a big game animal and one wants to kill cleanly ;out of respect for the animal so shooting at an animal at that distance by anyone is not ethical. A hunter should practice position shooting at various ranges with iron sights ,apply the 9" group kill zone rule and then determine what his range limitations are.