The Marine Corps is experimenting with M4 for combat arms units. 3rd Battalion, 2nd Marines was issued all M4 last year to see how they work out. So far the results have been mixed. There have been several complaints made by the recipients of the M4
-The RAS heated up too much, most Marines preferred using the stock M4 hand guards.
-Carrying handle, when used, came loose easily and didn't hold a zero
-Sights used on the RAS didn't hold zeros well, when sights were removed and remounted they had had to be rezerored.
-Left handed shooters had a lot of malfunctions because of the gun fighter slings issued with the weapon, brass would hit the sling and bounce back into the ejection port.
-Because of the shorted sight radius, the less skilled shooters had more problems qualifying with iron sights.
-The common tendency was to hit high. There were a few small others.
-Stock and cheek weld was down right not liked by the Marines use to the M16.