Take look at the Nikon Monarch with their target turret kit.

Monarch 4-16x42 with a MilDot/BDC reticle will run from mid-$300s on up on gunbroker (cheapest I saw was $369 for a new one with mildot reticle)

Target turret kit is another $70 for places like Opticsplanet.net

http://www.opticsplanet.net/nikon-mo...-knob-kit.html

Leupold AR scope is another good option. I prefer a scope with the side focus, but hear good things about this scope.