Atrius Launches X-Series FRS for MCX, MPX, and Rattler

Atrius launched the X-Series ambidextrous forced reset selector for SIG gas-piston guns. It is priced at $299 and named for MCX, MCX-Spear LT, MCX-Rattler LT, Sig's MCX-Spear LT Upper AR Conversion Kit, MPX PCC, MPX-K, and MPX Copperhead.

The part is a selector, not a complete trigger and not a locking-bar FRT. Positions are Safe, Semi, and Super-Semi. The lever uses the original Atrius ambi switch, with a new FRS lever sized for MCX and MPX product lines. Construction is heat-treated 4140 steel with a corrosion-resistant black nitride finish, made in the USA. Atrius calls the install drop-in, with no modifications and no trigger cutting.

Print Shoot Repeat and Lilly Tactical showed early units on video in August 2026. The X-Series kit is the selector plus a trip base and trip clutch, with a named host list and a price.

Atrius names the X-Series for MCX, Spear LT, Rattler LT, the Spear LT upper conversion kit, MPX PCC, MPX-K, and MPX Copperhead. The Copperhead callout matters because the AS Designs MPX slip trip kit excludes that host. Product photos show the selector in an FDE Spear LT, a black MPX-K, and a grey Spear LT. Regular X-Series stays on that list. Full-size Spear in .308 and 6.8x51 is a different gun and a different Atrius SKU: X-Series Heavy.

It drops into a factory SIG lower. Asked whether an early unit ran in OEM lowers, Print Shoot Repeat answered "Yep." MP5 and HK UMP forced reset conversions need a dedicated serialized lower. This one stays a parts swap.

X-Series Heavy is the Atrius forced reset selector for full-size MCX Spear in .308 and 6.8x51, also called .277 Fury. It is a separate SKU from regular X-Series. Atrius prices it at $319.

The kit uses a trip base and a trip clutch, plus a new FRS lever sized for Spear Heavy only. That lever does not interchange with other Atrius FRS models. Construction is heat-treated 4140 steel with a black nitride finish. Positions are Safe, Semi, and Super-Semi. Atrius calls the install drop-in, with no trigger cutting, on mil-spec MCX triggers that do not have a U-notch or relief cut.

Maker fitment is MCX Spear .308 and MCX Spear 6.8x51 only. MPX, Rattler, and Spear LT stay on regular X-Series. The tested alternate on that host is the AS Designs Heavy slip trip kit with Arc-Fire V2.

Atrius Development X-Series Heavy Ambidextrous Forced Reset Selector

Full-size MCX Spear in .308 or 6.8x51 / .277 Fury that wants an Atrius selector instead of the AS Designs Heavy stack

$319

Ambi forced reset selector with a trip base and trip clutch for full-size MCX Spear .308 / 6.8x51

Ambidextrous Safe, Semi, Super-SemiTrip base and trip clutch for full-size MCX SpearHeat-treated 4140 steel, black nitrideDrop-in; no trigger cutting

Pros

• +Purpose-built trip system and Spear Heavy lever for the full-size .308 / 6.8x51 Spear
• +Ambidextrous Safe, Semi, and Super-Semi without cutting the trigger
• +4140 steel with black nitride, made in the USA

Cons

• −Not the regular X-Series; does not fit MCX, MPX, Rattler, or Spear LT
• −Not the G-Lever; no extra Geissele or reduced-hammer compatibility
• −Cam lever is unique and incompatible with other FRS models

Material: Heat-treated 4140 steelFinish: Corrosion-resistant black nitrideThrow: 90-degree indexingPositions: 3-position (Safe/Semi/Super-Semi)

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

That is why the established path has been an adapter, not a selector by itself. AS Designs sells a slip trip kit that supplies a heat-treated 4140 steel slip trip and a 7075-T6 guide, then an Arc-Fire or AR Super Safety runs on top of it. Our SIG MCX forced reset trigger install guide still covers that stack, including the Heavy kit for .308 and .277 FURY.

Atrius solved it with its own trip parts. Regular X-Series and X-Series Heavy both add a trip base and a trip clutch for gas-piston platforms. Regular X-Series uses a new FRS lever dimensioned for MCX and MPX product lines. Heavy uses a different lever sized for Spear Heavy only.

The install sequence is: install the X-Series like any safety selector, install the fire control group, insert the trip base on the left side of the receiver and marry it to the rear trigger pin, insert the trip clutch onto the trip-base track and push it forward, press the trip base down so it sits under the rear takedown pin, confirm the trip has not shifted, then install the upper and dry function check.

We have not put rounds through a production X-Series or X-Series Heavy. The stack we have run on an MCX, and the tested alternate we still recommend for hosts we have shot, is an AS Designs slip trip kit matched to your caliber with an Arc-Fire V2 selector on top. Two parts, two purchases, and the configuration our MCX install guide is written against.

The trip kit is the caliber-specific half. The standard kit covers the MCX in 5.56, 300 BLK, and 7.62x39 across all generations, with early guns sometimes needing the factory safety latch removed. The MPX and MPX-K use their own kit. A .308 or .277 FURY Spear uses the Heavy kit, which ships the dedicated Spear transfer lever and runs the Arc-Fire V2 only. That remains the tested alternate on that host. Atrius now sells X-Series Heavy as a one-kit selector path for the same rifle.

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AS Designs MCX Slip Trip Kit

Standard-caliber MCX hosts in 5.56, 300 BLK, or 7.62x39 that need a reset surface the carrier does not provide

$174.99

Slip trip kit that lets an Arc-Fire or Super Safety forced reset selector run on the SIG MCX with a standard AR or Geissele MCX trigger group.

MCX-specific FRS slip trip adapter4140 heat-treated steel trip, 7075-T6 PTFE-coated guide with integrated lever blockerRequired to run an Arc-Fire or Super Safety selector on the MCXFits MCX 5.56, 300 BLK, and 7.62x39 (all generations)

Pros

• +The path to running a forced reset selector on the SIG MCX
• +Heat-treated 4140 steel trip and PTFE-coated 7075-T6 guide for durable, low-friction service
• +Integrated lever blocker removes a separate fitting step

Cons

• −Requires a separately purchased Arc-Fire selector or Super Safety; it does nothing on its own
• −Does not fit the MCX .308, .277 FURY, or the 9mm MPX
• −Not compatible with the Atrius Super Selektor, factory Timney triggers, or cassette and traditional FRTs

Material: 4140 heat-treated steel (slip trip, black oxide), 7075-T6 aluminum (guide, Type III hardcoat anodized with PTFE)Contents: MCX slip trip, slip trip guide with integrated lever blockerCompatibility: SIG MCX 5.56 / 300 BLK / 7.62x39, all generationsRequires: AS Designs Arc-Fire forced reset selector or AR Super Safety (sold separately); standard AR or Geissele MCX trigger group

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AS Designs Arc-Fire V2 Ambi Kit (0-90-180)

The selector that rides on top of the trip kit, and the only one the Heavy .308 / .277 FURY kit is spec'd to

$249.99

Second-generation Arc-Fire FRS with reduced drag and traditional 0/90/180 ambidextrous throw

0/90/180 throwAmbidextrousReduced drag V2 internalsActive Reset Clutch

Pros

• +Keep your existing quality trigger for semi-auto work
• +V2 internals cycle smoother than V1 with reduced drag
• +Traditional 180-degree throw to ARC is deliberate and unambiguous

Cons

• −Premium pricing versus conventional safety selectors
• −180-degree throw is slower than 45/90 variants for shooters who want a shorter sweep
• −Non-AR platforms require the separately-sold Slip Trip Kit

Part Number: ARCFIRE-AMBI-V2-0-90-180Material: 4140 Steel (selector), M2 tool steel DLC (ARC components)Finish: Black oxide (selector), DLC (ARC)Throw: 0° / 90° / 180° (Safe / Semi / ARC)

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The selector half keeps your existing trigger for semi. Arc-Fire V2 is ambidextrous with a 0/90/180 throw and works with mil-spec and super-safety-cut trigger groups; it is not compatible with the LaRue MBT-2S, CMC Single Stage, or Rise Armament RA-140. An AR Super Safety also runs on the standard MCX and MPX trip kits, but not on the Heavy kit and not alongside the MCX or MPX-specific Geissele trigger.

What the Atrius kits change, on paper, is the parts count and the trigger-group question: one selector plus its own trip base and trip clutch instead of an AS Designs adapter and a separate selector. That is a real simplification if it holds up.

The Spear LT upper conversion kit and the full-size Spear are different guns. Regular X-Series names the conversion kit. Full-size Spear in .308 and .277 FURY takes X-Series Heavy, or the tested AS Designs Heavy slip trip kit running an Arc-Fire V2.

Geissele owners want the G-Lever instead, a separate Atrius SKU whose lever blocker is what lets a stock SSA, SSA-E, or G2S run. The X-Series has no blocker. Its trigger path is a plain mil-spec fire control group, no U-notch and no relief cut, which is what Atrius means by drop-in. Cassette and drop-in triggers vary, so check yours against Atrius before you order.

Carrier velocity decides whether any forced reset system runs cleanly, and a short barrel with a suppressor on it is the hardest case an MCX presents. Our FRT tuning guide covers the gas, buffer, and spring side of that.

Atrius publishes a long restricted-state list on the X-Series product page, including California, New York, Illinois, Florida, Virginia, Washington, US territories, and international destinations. That list is the maker's shipping policy, not a statute chart. Check the state-legality tool for the legal layer, then read the product page before you order.

If you want the broader selector field, our super safety guide ranks forced reset selectors, and the safety-selector catalog is the current roster.

Atrius already sells four AR-15 selectors: a single-side FRS, a mil-spec-profile FRS for retro and clone builds, an ambidextrous FRS, and the G-Lever for unmodified Geissele two-stage triggers. X-Series and X-Series Heavy are the piston-gun rows, not extra AR-15 lever shapes.

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Atrius Development Forced Reset Selector

AR-15 hosts that already have a carrier trip surface and do not need the X-Series piston trip

$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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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.