Leupold makes a scope designed for the use on AR rifles. Its called the Mark AR. I havnt tried one yet, but I have never bought a Leupold that I didnt like, I would bet that it is a fine scope for what you are looking for. They come in 1.5-4x, 3-9x, 4-12x, and 6-18x. For the range you stated, I would go with the 3-9x(about $299) or the 4-12x (about $450). JMO.

Also Nikon makes one for the AR, its called the M-223, they are slightly cheaper than the Leupold. I dont know how good they are, I have had bad experinces with Nikon scopes and binoculars in the past. Worth looking at though, but in my personal experience I would stick with the Leupold.