Back in 2010, Mike Pannone and I were at the range doing some other shooting when we decided to do an impromptu dirt test with an AR. It was similar to the one I did in 2009, but with more ammo, and more dirt. I think we'll be doing another such test soon. Please forgive the poor production quality.
Here's Mike's narration:
The rifle is a 14.5” BCM M4 Spec barrel, spring was stock, buffer was H, trigger parts were factory. The rifle had fired 1200 rounds prior to the test with only 8-10 drops of MGL. It was not completely dry but was definitely dirty. I had used it for 5 NVG courses with a Federal Agency prior. The only malfunctions are at the end of magazines 5 (bolt over base on 28th round and then a double feed on 29 and 30) and 7 (failure to feed 30th round bolt over base) and those are both distinctly magazine caused. Feeding, chambering and locking as well as extraction and ejection are very positive. That is 210 rounds on a dirty gun under as close to combat/austere conditions as we can replicate for a desert environment. The mild overpressure in the upper receiver from the direct impingement system discharges the bulk of excess dirt and debris with the first shot and by shots 3 and 4 leaves at best only a light film of dirt on the face of the BCG.
The D/I system is far more reliable and far more tolerant to sand than the conventional wisdom would lead on. The D/I system actually aids in clearing out sand, dirt and debris and magazines are the weakest point of any operating system. Magazine 5 feeds too slow on the last 3 rounds and magazine 7 the last round. Both are symptoms of weak magazine springs and fouling due to sand.
Those were my crap training magazines and were shot through a dirty gun that was covered in sand with the ejection port cover open and up 7 times in a row.