(BUFDRVR) quoted:

"Some complain that standard U.S. military M16 and M4 rifles jam too easily in
Iraq's dusty environment. "

Ancedotal, but friends back from Afghanistan said they worked
fine in the apalling cement-like dust there.

"M4 with a Master Key" was their favorite. I'm a little
unclear on whether the Master Key is a 40mm or shotgun attachment, but
they used it for room entries.