I've been using an Aimpoint Comp M with 3' dot on my CAR15 for a while now. It's robust, bright enough for bright sun, and holds/adjusts zero well. The flip up lens covers and safety wired adjustment caps are a nice touch for in the field use. Battery life is longer with these than with the older generations of red dot scopes. I was a Tasco PDP2 man for many years, but have converted to these excellent scopes. Nothing else comes close, as far as I'm concerned. Forget the C-Mores, they're a big disappointment.
I recommend the GGG ring mount, which includes a spare battery compartment. I'm looking hard at the Mark Brown mount, which Gunsite uses on their CAR-15 conversions. It leaves the A2/A3 rear sight assembly in place, with the scope & rail occupying the area directly in front of the sight. The carry handle is milled off. Neat idea for keeping down size and leaving iron sights in place.