Mud Test: H&K 416/MR556
The H&K MR556A1, a piston-driven AR-15 variant, was subjected to a mud test. While it performed reliably with the dust cover closed, it failed to cycle when mud was introduced directly into the action with the dust cover open, underperforming compared to direct impingement AR-15s in similar conditions. This suggests piston systems may be less resilient to extreme fouling than DI systems.









