Maybe I missed this somewhere, but I'll put this HERE. I have multiple uppers of the 5.56 and 300BLK persuasion. I've tried red dots, prism scopes, and low powered variables. Each seem to have its pros and cons, but here are my suggestions:
Low powered variable scopes, with or without illuminated reticle are pretty simple. The lack of need for battery power is a plus. True 1x is best for both eyes open shooting, but not necessary. First (or front) focal plane means that the reticle is magnified as the power goes up, but relative distances between the potential aiming points remains constant. Prices vary considerably, but the Weaver Kaspa 1.5-6x is an excellent value for the price. If $$ were no object, I would opt for a SWFA 1-4x or 1-6x http://swfa.com/HD-C12503.aspx
Prism scopes are more compact than low powered variables, but far less common. The Burris AR series may be the most commonly known, but the Primary Arms units are (IMO) better and less $$. Reticles are etched, most are 5.56/308 compatible, but PA has 300BO/7.62x39 units as well. The PA units I have observed are extremely clear. Primary Arms is DIRECT, meaning that they have the optics manufactured to their specs and sell direct, so no distribution middlemen to drive up costs. http://www.primaryarms.com/scopes/c/140100/filter/100000001298eq100000001442/
Red dots vary from really cheap to really expensive. These are the(generally) no magnification units (optional magnifiers) that mostly use batteries and are dead in the water without. Dot sizes vary, battery types vary, and so does size. There are both reflex type (Burris Fastfire and clones) and the optical tube variety (think Aimpoint, Tasco Propoint) that have been around for quite some time. Trijicon Acogs are very expensive. The Lucid HD7 is pretty interesting if you don't need magnification, and are OK with a battery powered reticle. In fact, its a multi-reticle unit with automatic automatic brightness compenstion, and available 2.5x magnifier. http://www.primaryarms.com/l-hd7/p/l-hd7/
Low powered variable scopes, with or without illuminated reticle are pretty simple. The lack of need for battery power is a plus. True 1x is best for both eyes open shooting, but not necessary. First (or front) focal plane means that the reticle is magnified as the power goes up, but relative distances between the potential aiming points remains constant. Prices vary considerably, but the Weaver Kaspa 1.5-6x is an excellent value for the price. If $$ were no object, I would opt for a SWFA 1-4x or 1-6x http://swfa.com/HD-C12503.aspx
Prism scopes are more compact than low powered variables, but far less common. The Burris AR series may be the most commonly known, but the Primary Arms units are (IMO) better and less $$. Reticles are etched, most are 5.56/308 compatible, but PA has 300BO/7.62x39 units as well. The PA units I have observed are extremely clear. Primary Arms is DIRECT, meaning that they have the optics manufactured to their specs and sell direct, so no distribution middlemen to drive up costs. http://www.primaryarms.com/scopes/c/140100/filter/100000001298eq100000001442/
Red dots vary from really cheap to really expensive. These are the(generally) no magnification units (optional magnifiers) that mostly use batteries and are dead in the water without. Dot sizes vary, battery types vary, and so does size. There are both reflex type (Burris Fastfire and clones) and the optical tube variety (think Aimpoint, Tasco Propoint) that have been around for quite some time. Trijicon Acogs are very expensive. The Lucid HD7 is pretty interesting if you don't need magnification, and are OK with a battery powered reticle. In fact, its a multi-reticle unit with automatic automatic brightness compenstion, and available 2.5x magnifier. http://www.primaryarms.com/l-hd7/p/l-hd7/