The HK MR556A4 meets the Moondust test.

Published on October 21, 2025
Duration: 20:17

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.

Quick Summary

The HK MR556 A4 initially passed a moondust test with its port cover closed, firing 15 rounds of Federal M193. 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 malfunctions persisted, leading the presenter to favor direct gas impingement systems.

Chapters

  1. 00:10Introduction to Moondust Testing
  2. 00:35Focus on Short-Stroke Gas Piston ARs
  3. 00:51Introducing the HK MR556 A4
  4. 01:10Ammunition: Federal M193
  5. 01:32Moondust Preparation and Description
  6. 02:01Test Protocol: Port Cover Closed
  7. 02:24Test Protocol: Port Cover Open
  8. 03:01Gunsmithing Career Advice
  9. 03:38Lubrication Best Practices
  10. 04:17Gas System Settings and Port Cover Closed Test
  11. 04:52First Firing Sequence (Port Cover Closed)
  12. 05:17Second Firing Sequence (Port Cover Closed)
  13. 06:37Transition to Contaminated Bolt Carrier Group Test
  14. 07:03Simulating Field Disassembly and Contamination
  15. 07:33Reassembling Contaminated Bolt Carrier Group
  16. 07:50First Firing Attempt After Contamination
  17. 08:11Chambering Another Round
  18. 08:23Subsequent Firing Attempts
  19. 09:08Attempting to Lock Bolt Open
  20. 09:53Initial Reaction to Test Results
  21. 10:01Analysis of Bolt Carrier Group Failures
  22. 11:03Internal Inspection of the Rifle
  23. 13:15Examining the Gas System
  24. 13:43Quick Field Maintenance and Reassembly
  25. 15:32Final Firing Test After Cleaning
  26. 16:11Presenter's Disappointment and DI Preference
  27. 17:24Magazine Condition Check
  28. 18:21Comparing HK 416 to M4/M16
  29. 19:11Concluding Thoughts and Viewer Engagement

Frequently Asked Questions

How did the HK MR556 A4 perform in the moondust test with its port cover closed?

Initially, the HK MR556 A4 performed well in the moondust test with its port cover closed, successfully firing 15 rounds of Federal M193 ammunition without any malfunctions. This indicated good initial resilience to external contamination in its gas system.

What happened when the HK MR556 A4's bolt carrier group was intentionally contaminated with moondust?

After the bolt carrier group was deliberately rolled in moondust and reassembled, the HK MR556 A4 began to short-stroke. It failed to cycle fully to the rear, preventing it from picking up and chambering new rounds, leading to repeated failures to fire.

Did a quick field cleaning resolve the HK MR556 A4's malfunctions after moondust contamination?

No, a quick field cleaning and lubrication did not fully resolve the malfunctions. While the rifle cycled all rounds in the magazine, it still failed to lock open after the last round, indicating persistent issues from the contamination.

What is the presenter's preference regarding AR-15 gas systems after this test?

Based on this test and prior experience, the presenter reiterates a preference for direct gas impingement (DI) systems over short-stroke or long-stroke gas piston systems in AR-15 platforms, citing perceived superior reliability.

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