I have a rifle class coming up soon. And because I don't want to look like a donkey any more than I have to, I spent this afternoon making sure that my rifle is zeroed perfectly, both with it's iron sights and the two different optic units that I'll be bringing with my.
My Bushmaster carbine, with an A.R.M.S. #40 BUIS (Back Up Iron Sight) and a Grip-pod forward vertical grip that's also a bi-pod. (This is one of the coolest AR accessories out there, folks. If you don't have one, get one.)
An Aimpoint Comp ML4 zero-magnification red-dot on a LaRue quick-detach tactical mount.
And a Trijicon ACOG 4x scope, also on a LaRue QD mount.
Note: Neither Bushmaster nor LaRue Tactical, Aimpoint, Trijicon, A.R.M.S or Grip Pod gave me anything to flog their stuff; I just own and use it because I really like it.