Is this a SHTF carbine type of AR?
If so, you want a top-notch optic and won't get it for a couple of hundred dollars.
Also make sure to have good back-up iron sights and that they are zeroed.
Look at the Aimpoint Comp M2 or M3-series sight and LaRue cantilever mount.
http://stores.homestead.com/Laruetac...tail.bok?no=18
http://stores.homestead.com/Laruetac...tail.bok?no=19
Batteries will last a long long time, approx 50,000 hours or more.
You don't have to turn it off.
I have an early Comp M mounted on a Ruger Mk3 22 pistol. While it may be a bit heavy for that application it works very well.
Allen