The HK416/MR556 (Piston) is less reliable in dirt than the AR15 (Direct Impingement).
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The HK416/MR556 is a piston variant of the AR15 weapon family, and while that helps with some things – like suppression, it actually performs worse in dirty conditions as the semi-DI system of the AR15 blows dirt out of the action, unlike the short stroke piston system.