Atrius FRS for SIG MCX Confirmed Incoming, No Date Yet

Atrius Development is building a forced reset selector for the SIG MCX, and it runs in factory lowers. Those are the two confirmed claims, and they come from two sources posted several days apart. Print Shoot Repeat published a reel captioned "FRS MCX!" tagging Atrius directly. Lilly Tactical followed on August 12, 2026 with a shooting video captioned "ATRIUS FRS for SIG MCX IS COMING!!!!!! GET READY!!!!!!" carrying the same tag, which cleared 17,000 likes inside a day.

The OEM lower answer came from Print Shoot Repeat himself. Asked in his comments whether the selector works in the OEM lowers, he replied "Yep." That is the single most useful detail released so far, because it rules out the failure mode that makes forced reset conversions expensive on other platforms. The MP5 and the HK UMP both need a complete dedicated lower to run forced reset, which turns a parts purchase into a serialized-component purchase. An MCX selector that drops into the factory lower stays a parts purchase.

Nothing else has been released. There is no price, no ship date, no part number, no compatible model list, and no install walkthrough. Atrius has not posted an announcement of its own, and atrius.dev currently lists four AR-15 selectors with no MCX entry and no coming-soon placeholder. Two shooters with early access filmed the part working, and that is the whole of the public record right now.

That is still news worth acting on if you own an MCX, because it signals the first purpose-built Atrius path onto a platform that has been awkward for forced reset hardware since the category started.

The MCX bolt carrier does not present the reset trip surface a forced reset selector needs. An AR-15 selector like the Atrius FRS works because the carrier reliably strikes a cam on the selector during its return stroke, which drives the trigger reset. The MCX carrier geometry does not do that, so a bare selector dropped into an MCX lower has nothing to trip against.

This is why the existing MCX solution is not a selector at all. AS Designs sells a slip trip kit that supplies a heat-treated 4140 steel slip trip and a 7075-T6 guide with an integrated lever blocker, which together create the reset path the carrier is missing. The Arc-Fire selector or an AR Super Safety then runs on top of that adapter. Our SIG MCX forced reset trigger install guide covers how that stack goes together and where it bites.

The interesting question is whether Atrius solved this with its own adapter or found a way to make a selector that trips off the stock MCX carrier. The first is straightforward and puts them head to head with AS Designs. The second would be a genuinely new piece of engineering and would explain why the part is being teased on video before it exists on a product page.

Model coverage is the question that decides whether this matters to you, and it is the one still open. The comment threads filled with the same asks: does it run on the Rattler, does it run on the Rattler LT in 300 BLK, does it run on the MPX, does it cycle reliably suppressed, and does it need a trigger bar. Every one of those got the same answer from Lilly Tactical, which was to ask Atrius directly.

Those are not variations on one question. The standard MCX in 5.56, 300 BLK, and 7.62x39 shares a carrier family. The Spear in .308 and .277 FURY runs a heavier action that needs a different reset path, which is why AS Designs ships a separate heavy kit for it. The MPX is a different gun entirely and takes its own dedicated kit today. A part that covers all four would be a first. A part that covers only the standard-caliber MCX would still be useful, and would leave Spear and MPX owners exactly where they are.

Suppressed cycling is the other one to watch. Forced reset hardware is sensitive to carrier velocity, and the MCX is very commonly run short and suppressed, which is the hardest case. Our FRT tuning guide covers the buffer and carrier-weight relationship that governs this on AR-pattern guns, and the same physics applies here.

I have a personal stake in how this lands. My own MCX runs an Arc-Fire with a Geissele trigger, which is the established path on this platform, and it has never reset consistently. Chasing that has meant living with the trip geometry rather than the selector, which is exactly the friction a purpose-built drop-in part is supposed to remove. Whether the Atrius unit actually removes it depends entirely on the fitment list nobody has published yet.

Atrius currently sells four AR-15 selectors: a single-side FRS, a Mil-Spec profile FRS aimed at retro and clone builds, an ambidextrous FRS, and the G-Lever ambi built to drop into stock Geissele triggers without cutting. The single-side unit below is the baseline the MCX part would be derived from.

In Stock

Atrius Development Forced Reset Selector

$199.00

Drop-in forced reset selector with three firing modes at a lower price point

90-degree throwRight-hand onlyHeat-treated 4140 steelThree firing modes

Pros

• +Same forced reset performance as ambi version at $169.99
• +Drop-in installation with Mil-Spec fire control groups
• +Heat-treated 4140 steel construction

Cons

• −Right-hand only - no ambidextrous selector
• −May require minor fitment during installation
• −Legal status varies by state - verify local regulations

Material: Heat-treated 4140 steelFinish: Corrosion-resistant coatingThrow: 90-degreePositions: 3-position (Safe/Semi/Full-Semi)

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Pricing on an MCX part is unannounced. The current single-side selector runs $169.99, and an MCX version carrying additional adapter hardware would reasonably land above that. Anyone quoting a firm number today is guessing. If you want the broader field while this develops, our super safety guide ranks the current forced reset selectors across platforms.

Forced reset selectors are federally available following the DOJ settlement, and the administration has confirmed it will not appeal. State law is the binding constraint, and several states prohibit forced reset hardware outright regardless of federal status. Check your own state before ordering, and re-check before shipping to a new address. This is the layer that has moved most over the last two years and the layer most likely to move again.