The AO (aside from serving to help minimize parallax) also serves a target focus function. While some have never noticed it, the next time you're at the range, set your AO for 100 yards and observe how clear a 100 yard target is. Then, without moving the AO adjustment, look at something 300 yards away. You'll see that it's not nearly as "focused" as the 100 yard target was. The reverse is also true. An AO set for 250 will provide a sharp target image at that range, but not so clear at 100.
I've found with big game rifles, or on rigs that are used against solitary (non-colony/herd) game animals where the range can vary wildly, it's better to have a non-AO scope, which provides a sharp focus over a greater latitude of range than does an AO equipped scope.
Mike
