The Trijicon ACOG TA31 is the toughest combat scope available and it so tough and reliable that the USMC uses is on the M16 and M4. It is simply the best sighting system for combat in all lighting conditions. It is a 4 power scope that provides a bright sight picture with an illuminated reticle.
Watch this demonstration of the Trijicon ACOG TA31RCO combat optical sight. The features are very impressive.