The basic M16 design including all variants appears to be made more for the civilian market than the military. I mean, aside from all else, civilians simply would not be out shooting in a sand storm...
Dumb question: Ever had a look at the Marlin 38A 22 lever-action? A grain of sand would have to really work at getting into that one.
Question is, do you really need the full auto mode for an assault rifle? My own vision of an ideal general army rifle would be something somewhere between the 38A and the new Henry rifle, 26 caliber or thereabouts, rimfire, 65 - 80 gr bullets at something like 2500 fps, detachable mags, and short-throw lever like the Brownings, but with that same covered ejection port like the 38A. Weight would be similar to that of the Henry rifle, i.e. three or four pounds, no need for the weight of the 38A in today's world. You'd clean it in five seconds by pulling a bore snake through the barrel.
Does that idea appeal to you at all?