Take any M16, oil it well, and allow sand into the action and see what happens
The U.S. Army did exactly that in 2007. In fact they did it twice in 2007. The worst result for the M4 after 60,000 total rounds through 10 rifles in simulated sandstorm conditions with 600 rounds between any kind of cleaning was a 1.4% stoppage rate. The M16 performed even better.
For comparison, new modern designs (SCAR, XM8, HK416) were able to improve on that an additional 0.8-1.0% (going from memory so numbers may be off a few tenths)