MARC Won't Fire or Reset in Active Position — Fitting Guide

This guide covers the documented fitting procedure for two-stage triggers (Larue MBT-2S, Geissele SSA/SSA-E/G2S, Schmid) that don't run with the MARC out of the box. Check the compatibility chart first to see what your specific trigger typically needs.

Disclaimer: These procedures record our team's documented experiences. Individual triggers vary — yours may need more or less material than described. Modification is at your own risk and may void the trigger manufacturer's warranty. Copperhead Safety is not responsible for damages, lack of function, loss of warranty, or unsafe firearms resulting from modification. Do not proceed if you are not willing to accept these risks. We will never instruct or advise anyone to modify a sear surface.

Understand the mechanics first

Three parts interact: the trigger tail, the disconnector, and the hammer.

A two-stage trigger's longer pull means the trigger tail rises higher in the lower before the hammer drops. If the tail rises far enough to hit the bottom of the MARC's axle, the hammer can't release. Symptoms: a much heavier-than-normal pull in semi-auto, and no break at all in the active-reset position.

Each fix below can create the need for the next one — which is why you work in this exact order, removing the minimum material at every step.

Step 1 — Trigger-tail relief

1. Remove a small amount of material from the top of the trigger tail with a fine file or sandpaper. Typical total removal is under 0.5 mm. On Geissele two-stage units, very light sanding and polishing is usually enough.
2. Reinstall and hand-function check. Repeat tiny passes until the hammer releases in the active-reset position with no more pull weight than semi-auto.
3. Slight rounding of the top rear edge of the tail can help.
4. With proper clearance, the MARC's spring should not move much or bind during the trigger pull.

Safety check at every pass: function-check on Safe. Excessive tail removal can delete the surface that interfaces with the safety, making the weapon capable of firing on Safe. If the fix requires preserving safety engagement, remove less material from the right (ejection) side of the tail — that side interfaces with the safety surface. A hump should remain on the right side if this style of removal is needed.

Step 2 — Test lever function

1. Pull the trigger so the hammer releases; hold pressure on the trigger.
2. Reset the hammer by hand and push the MARC's cam lever to the rear.
3. If the trigger moves and resets easily — you may be done. Go shoot it.
4. If the trigger moves some but does not reset — go to Step 3.

Why this happens: lowering the tail height reduces how far the cam lever can push the trigger shoe down on the carrier's return stroke. On some triggers the tail is no longer pushed far enough to reset.

Step 3 — Disconnector relief

The disconnector surface determines when the hammer is allowed to reset. Shaving the lower forward edge of the disconnector shortens the travel required to reset the trigger.

• Work carefully, gently, gradually. On some MBT-2S units there's a slight ridge in this area — removing that ridge alone is often enough.
• Remove too much and the disconnector grabs the hammer too early — the trigger won't reset properly. That failure pushes you to Step 4.

Step 4 — Hammer timing (last resort)

If hand function is good but the gun stops working with the upper installed, the hammer may be getting pushed too low by the BCG (seen where a lot of disconnector material was removed). The documented fix: cut the top-forward corner of the hammer, which delays when the hammer engages the disconnector.

Know your hammer before cutting:

• Larue MBT-2S hammers carry plenty of mass — they tolerate this cut well.
• Geissele and other smaller hammers can develop light strikes on hard primers after losing this material. If that happens, see the light-strike guide.

The rule that runs through all four steps

Minimum material, tiny passes, function check (including on Safe) after every pass. These three surfaces — tail, disconnector, hammer — properly fitted, resolve most MARC compatibility issues.