MR556, MR223, HK416, BRN-4 BCG - All versions compatible (A1, A4, etc.) with no modifications from stock
To install ARC-Fire, you need an AR-15 mil-spec FCG (you can skip the trigger if you're getting parts individually) and our precut trigger that comes with the ARC-Fire. You can also use Geissele MR556, but you will need to perform the super safety cut on the trigger or purchase one pre-cut. If you use the Geissele MR556, you don't need the lever blocker as the hammer is full sized.
In our testing, you don't have to adjust the gas vent screw, but we do most of our testing suppressed. HKParts suggests 1.8mm vent screw for MR556 A4 and HK416 A5 models.
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