Take a look at the Burris AR332. It's a nice, compact, fixed 3x prism sight with a 5.56 BDC reticle. The reticle can be illuminated in red or green (at multiple brightness settings in either color), and when turned off or the battery goes dead, you still have a black reticle. It's also quite affordable at around $300 and has integral mounting capability for either a rail or a standard AR carrying handle.