From my own posting on another forum:
"There are three ways to deal with sand/dust in the design of a military rifle.
The first is to keep it out. The M16 design tries to do this with a closed action and a dust cover. This approach does not work because while the amount of crud getting in is reduced, what does get in can't be gotten out easily.
The second is to make the rifle easy to clean. The U.S. M1 rifle is a good example of this approach. The locking lugs and seats are easy to get to, even with a finger, to clean out dirt. The trigger group is easy to remove and clean with cleaner, water, or by simply blowing on it. The AK-47, also a dual lug rotating bolt system, is similarly easy to clean, though not as easy as the M1 since the cover has to be removed.
The third approach is to design the rifle to simply ignore dirt. While not a rifle, the M3 SMG is a good example. It can take in huge quantities of sand and dirt without interfering with the mechanism. The only rifle close to it is the AR-18/180, but it also uses the multi-lug bolt of the AR-15/M16.
IMHO, the third approach has the best chance of making a "sand proof" rifle, but the multi-lug bolt is a weak point. "
I was flamed by an M16 fan who claimed the M16 is easy to clean and offered to show me how. Nice of him. It is not hard, but you need special equipment; that toothbrush won't do.
Any piece of machinery can be stopped by the wrong environmental conditions. The designer's problem is to keep stoppages to an absolute minimum without requiring the user to be in a constant care mode.
Jim