Hawke Vantage 3-9x40mm AO All Around Use Riflescope- It's the AO feature that makes this scope so suitable for muzzleloading, shotgun slug, centerfire, rimfire, and even airgun rifle hunting. "AO" means Adjustable Objective... Which means you can dial out parallax error from 10 yards to infinity. Parallax error is how much the crosshair moves when you move your head position. Most rifle scopes are hard set for 100 yard parallax. When you aim with those scopes at shorter or longer distances, a simple change in the position of your head resting on your buttstock can throw off your shot by a few to several inches. - Try this experiment with a scope that doesn't have AO: Position your rifle on a shooting rest with the crosshairs on the bullseye. Now, without touching the rifle, look through the scope while you move your head from side to side. If you see the crosshair moving off the bullseye, that's parallax error. Now if your scope has AO, you can dial parallax out for any exact distance from 10 yds to infinity. So if you're hunting in the woods and shooting 40 yards, you can set your parallax to be error free at that distance. Or if you're on a beanfield aiming 150-200 yards, you can set your scope to be parallax free at that distance. For shooting the tightest groups, exact parallax adjustment is critical. But for hunting situations, you just set it to the general distance you expect to shoot and forget it... Its not complicated... Just ACCURATE!... THIS scope gives you that capability.