[Big Bronze Rim]

If you want unmagnified the Aimpoint PRO is hard to beat. For around $400 you get a super rugged optic with a quality mount. The battery life is excellent (3 years) and you can just leave the dot on and it's always ready to go if it's a defensive carbine. If you want magnification, run a 3x magnifier on a QD mount.

http://www.aimpoint.com/us/products/all-products/product-singleview/product/PRO/