FRT Light Strikes: Diagnosis & Fixes — Bolt Bounce, Auto-Cam

A light strike — primer dented but no ignition — in a MARC build has three documented causes. Work them in this order, because the first is the most common and the cheapest to fix.

Disclaimer: the auto-cam fix below involves modifying the MARC's cam lever. As with all modifications documented in these guides: your own risk, minimum material, function check constantly. Copperhead Safety is not responsible for damages, lack of function, loss of warranty, or unsafe firearms resulting from modification.

Cause 1 — Bolt bounce (most common)

When the carrier returns to battery, a too-light buffer lets it bounce off the barrel extension. If the hammer falls during the bounce, the primer takes a soft hit.

Fix: heavier, dead-blow buffer weights and gas tuning. Run H1/H2 (16" mid-length spec), H2/H3 suppressed. Full detail: buffer requirements guide.

Rule out bolt bounce before touching anything else.

Cause 2 — Auto-cam (mostly two-stage triggers, Geissele two-stage most often)

What it is: the surface on the MARC's cam lever just behind the cam surface is rounded. Its job is to block the trigger from rising too high before the cycle finishes — a forced safety that assists timing and prevents hammer-follow failures. Side effect of the curve: if the trigger is pulled hard and held, the trigger itself can push the cam lever forward early — before the bolt carrier would have moved it. The trigger then breaks early in the cycle and the hammer starts falling before the gun is in battery.

Why it shows up as a light strike, not a detonation: on a standard AR-15 bolt, the firing pin cannot contact the primer until the bolt is in battery. The hammer falls, rides the bolt home, and hits the firing pin with most of its force already spent. Result: light strike (often with hammer follow).

How to identify: light strikes that persist after buffer and gas are correct, on a two-stage trigger — Geissele two-stage units show this more than others.

The documented fix: carefully modify the cam lever with a Dremel or a very hard file, in the small ~1/8" area directly behind the cam hump, so the cam surface profile resembles a knuckle instead of a long round curve. This removes the forward bias.

• Many levers and many guns will not need the whole area removed. Light, even passes.
• Hand-function check frequently. Correct dimensions: the trigger can be pulled moderately during hand function, and the cam lever "jumps" forward when it's near vertical.
• Operate the lever slowly and watch exactly where it releases the trigger tail and lets the trigger break.

Cause 3 — Reduced hammer mass (only if you cut the hammer)

If you cut the top-forward corner of the hammer during trigger fitting (Step 4 of the fitting guide), smaller hammers — Geissele in particular — can go light on ammunition with hard primers. Larue MBT-2S hammers carry enough mass to tolerate the cut.

Fix options: run ammunition with softer primers, or replace the hammer and redo the fitting sequence with less disconnector removal so the hammer cut isn't needed.

Quick diagnostic table