The fact of the matter is that the M-16 got some of our troops killed in the invasion of Iraq because it could not fire with sand particles in the receiver. One young private whos M-16 was only partially disabled was able to take out an entire Iraqi mortar crew by firing a shot, ducking down, and unjamming his rifle, then firing again. Apparently he was an expert marksman.
So, 50 years after its invention, the M-16 still can't withstand the rigors of combat. People continue saying "Keep it clean and it works". Maybe. But being out in the field is an extremely dirty environment. Sand and snow environments are alot alike, the particles get everywhere. Same with mud.