Mil-Spec sights are quite literally ready for combat.
They have been certified for water resistance (some even for underwater use), shock tested, impact tested, and more. Sights that meet military specifications are usually night vision device (NOD) compatible.
Mil-Spec red dots include models from Aimpoint, Burris, Trijicon, and now Insight Technologies. These sights generally cost about $300 to $600.
If you wear glasses, the absolute best red dot sight for you is the holographic sight. Folks who wear glasses tend to see a "black halo" in any tube type sight, be it a red dot or a telescopic sight. Holographic sights eliminate this problem for most users.
Holographic sights project the dot onto a screen, not into a tube. This tends to give the operator a better field of view because the controls are in the base of the unit, not built into the tube, which adds to the diameter of the sight.
The most popular holographic sights are made by Bushnell, EoTech, and C More. EoTech offers Mil-Spec versions that are night vision capable. But don't overlook Bushnell and C More if you don't need night vision compatibility.
In some aspects holo sights are even more rugged than standard red dot rifle optics. You can break the lens on the sight and still use it.
Holographic sights range in price from $200 to $500, depending on the make and model that you purchase.
Magnifying Red Dots
Magnifying red dots do exactly what you think they do. They magnify the target image. These sights have magnification capability usually somewhere between 3x and 5x. They also have unique reticles, chevrons, triangles, circle dot, and other features that are only offered by specific manufacturers.
The market leaders among magnifying red dot sights are the Trijicon ACOG (Advanced Combat Optical Gunsight) and the Leupold Mark IV CQT.
It has been my experience that these sights are virtually indestructible. I know of a CQT that was dragged down a road in Baghdad when the M16 it was attached to dislodged from its weapons rack on the MULE four-wheeler the operator was driving. And I have seen photos of ACOGs that have been hit with rounds but still work.
The only drawback to these sights is you get what you pay for. They are a little bit pricey, ranging from $700 to more than $1,500.