SIG MCX Forced Reset Trigger Install (MCX, MPX, Spear)

Which Slip Trip Kit Your MCX or MPX Needs

Caliber decides the kit. A standard-caliber MCX runs the MCX Slip Trip Kit, a 9mm MPX runs the MPX kit, and a heavy-caliber Spear runs the Heavy kit, all from AS Designs. None of them force the reset on their own, though; you buy an Arc-Fire selector separately for that. And they aren't interchangeable. Grab the wrong one and its transfer geometry won't line up with your carrier. These kits are the selector path; if you'd rather run an actual FRT trigger, the Frankenstein TSBs further down are your adapter instead.

Two fitment traps to avoid. The standard MCX kit does not fit the MPX or the heavy-caliber Spear, and the MPX kit does not fit the MPX Copperhead or the MCX. The Heavy kit is the odd one out on the selector side: it is built for the Arc-Fire V2 and will not run an AR Super Safety, so do not plan a Super Safety Spear in .277 FURY. For a broader look at MCX upgrades beyond the trigger, see our SIG MCX accessories guide.

The Two Forced-Reset Paths for an MCX

Whichever way you go, the starting problem is the same. A forced reset device needs a trip surface on the bolt carrier to work against as the action cycles. A standard AR-15 carrier presents that surface; the MCX and MPX carriers do not. So you install an adapter that supplies it, and the adapter you pick decides whether you run a forced-reset selector or an actual FRT trigger.

Path one is the forced-reset selector. An AS Designs slip trip kit, a 4140 steel slip trip and a PTFE-coated 7075-T6 guide with an integrated lever blocker, supplies the reset surface, and on top of it you run a Super Safety or an AS Designs Arc-Fire. This is where the terminology trips people up: the Super Safety is a selector, not a trigger. It replaces the safety selector and keeps your existing MCX trigger for normal semi-auto fire, forcing the reset only in the ARC position. The Arc-Fire is AS Designs' own selector and works the same way. For how forced-reset selectors work in depth, see our Super Safety guide, and the FRT buyer's guide is the category hub across every platform.

Path two is an actual FRT trigger. Same problem, opposite device. Here the adapter is a Frankenstein Armory TSB instead of a slip trip kit: it supplies the surface a locking-bar forced reset trigger needs, so you drop in a real FRT rather than a selector on top of your factory trigger. The TSB-Lite ($100) covers the standard MCX and MPX family; the TSB-Heavy ($100) covers the .308 and .277 FURY Spear. Both are drop-in with included pins and no cutting, and both are explicitly not compatible with a Super Safety. Pick this path when you want a locking-bar FRT trigger; pick the slip trip kit and a selector when you want to keep your factory trigger feel in semi.

Removing the Firing Pin Safety Latch

On most MCX and MPX guns the firing pin safety latch has to come out for the forced reset to run reliably, and it is needed most on early-generation rifles. It interferes with the reset timing a forced reset selector depends on. AS Designs states the safety latch may need to be removed in early generations across the MCX, MPX, and Heavy kits. Whether your specific gun needs it removed is generation-dependent, so verify against your own rifle before you start.

1. 1.
   Clear it first. Drop the mag, lock the bolt to the rear, and both look and feel for an empty chamber. Nothing else happens until the gun is cold.
2. 2.
   Split the receivers. Push both takedown pins, lift the upper off, and pull the bolt carrier. That gets you into the fire control, where the safety latch sits.
3. 3.
   Find out if your gun even needs it. Early guns almost always need the latch out; plenty of later production resets fine with it in place. Figure out which one you have before you start pulling parts on spec.
4. 4.
   Pull the latch. If yours needs it, remove it per AS Designs' instructions for your generation. It is a factory safety part, so treat it as a deliberate call and hand the job to a gunsmith if you are not sure.
5. 5.
   Reassemble and function-check. Fit your adapter and device, either a slip trip kit and selector or a Frankenstein block and an FRT trigger, put the gun back together, and dry-cycle while holding the trigger to feel the reset and confirm the disconnector still catches in semi. Run it dry through every position before live fire.

Selector Choice and the Reset-Energy Problem

The canonical reset-failure mode on a SIG MCX FRT build is a trigger spring that is too light. On a gas-adjustable Spear LT, a reduced-power Geissele trigger spring can leave too little energy in the system to drive the trigger forward, so it fires a round or two in semi and then goes dead. The gas-adjustable action and the ARC reset mechanism both draw off some of the energy the trigger needs to return, and a light competition spring loses that margin. A mil-spec-power trigger spring resets more reliably and is the safe default unless your gun is deliberately over-gassed.

Do not read that as universal. This is a tuning shortfall, not a defect, and it does not hit every build. In our own hands-on testing a Geissele SSA-E ran a Spear LT 11.5 for 400 rounds in ARC mode without a dead trigger, which is exactly why the fix is "start mil-spec" rather than "never run a Geissele." The reset-energy balance between carrier mass, gas setting, and spring rate is the same lever set covered in our AR-15 FRT tuning guide, and the Arc-Fire selector that anchors this build was tested on a Spear LT in our AS Designs Arc-Fire review.

The MPX changes the selector-and-trigger advice outright. AS Designs recommends running a generic mil-spec AR fire control group rather than a Geissele on the MPX, because the platform is known to damage trigger components under forced reset use. So on an MPX the default is a mil-spec FCG plus the MPX slip trip kit and an Arc-Fire or Super Safety, not a premium two-stage trigger.