MR762, HK417 - 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, but you need to drill the hammer's pin hole oversized to ~0.165". You can also use Geissele MR762, which is what we recommend, but you will need to perform the super safety cut on the trigger or purchase one pre-cut. If you use the Geissele MR762, you don't need the lever blocker that normal Geisseles need 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.
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