The HK MR556A4 meets the Moondust test.
This video details a "moondust" (fine sand/dirt) test on the HK MR556 A4, a short-stroke gas piston AR-15. The rifle initially performed well with the port cover closed, firing 15 rounds without issue. However, after intentionally contaminating the bolt carrier group with moondust, the rifle began to short-stroke, failing to chamber new rounds. Despite a quick field cleaning, the failures persisted, leading the presenter to express surprise and disappointment.












