Springfield Kuna FRT Guide: Lower, Trigger & Install

The Nexus lower is the gate to the entire AR-15 trigger aftermarket on a Kuna, and it is the only product that does this job. It replaces the polymer factory lower with a billet aluminum unit that accepts a standard AR-15 trigger, safety selector, and pistol grip. It ships with a fully ambidextrous bolt release, a captured takedown pin, a safety selector spring shim, and the magazine release. No FFL is involved, because on the Kuna the upper receiver is the serialized part.

The two variants differ only in the magazine release. The paddle version places it at the front of the trigger guard in AK and Scorpion style, from $349. The push button version costs slightly more and uses an AR-style release on the left side. Both behave identically in a forced reset build, so choose the manual of arms you already run.

Magazines change with the conversion. The factory 30-round Kuna magazines no longer fit, and the lower feeds Magpul PMAG EV9 Scorpion magazines; per Nexus, other Scorpion EVO pattern magazines do not feed reliably. The lower is also made to order, so order it before anything else in the chain.

Nexus sells two trip kits, and they are not interchangeable: each one supports a different set of triggers or selectors, so pick the trip before you buy anything to go with it. Both require the Nexus lower and neither works in the OEM Springfield lower.

The Rare Breed FRT L3 Trip ($99.99, SKU K-2-004) is the trigger path. Nexus lists it as designed for the Rare Breed FRT-15L3 and Delta Team Tactical (DTT) drop-in forced reset triggers, the two families it was cut around. The Triggered Company Disruptor also runs on it for most builders, with the caveat covered in the next section. The Super-Safety Trip ($79.99, SKU K-2-003) is the selector path. It works with any standard Super-Safety as well as the Atrius Super-Selektor, and Nexus reports it also works with the AS Designs selector, though it notes that rests on customer feedback rather than its own testing.

Nexus sells both trips as separate products rather than as part of the lower, which allows a staged build: install the lower, run the gun as a semi-auto with a conventional trigger, and add the trip and forced reset parts later.

Two triggers are worth considering, and they trade fitment certainty against pull weight and price. The Triggered Company Disruptor is the better trigger on paper: a 3.75 to 4.1 lb pull against the FRT-15L3's 5.5 to 6 lb in reset mode, at a little over half the price, as a drop-in cassette with anti-walk pins. Most Kuna builders running one report it working well.

The caveat is timing. Nexus cut the K-2-004 trip around the Rare Breed L3, and the Disruptor's timing and housing differ enough that results have been mixed, with some builders hitting reset problems. Nexus has since revised the trip to run the Disruptor better, so if you are building around one, confirm with Nexus that you have the current trip revision rather than an early one. The Rare Breed FRT-15L3 remains the lowest-risk fitment because it is the trigger the trip was designed for; it costs more and pulls heavier for that certainty. Nexus sells neither trigger, so either is a separate purchase.

A forced reset trigger and a Super-Safety are different parts. An FRT is a self-contained trigger group with the reset mechanism inside it. A Super-Safety is a selector: a cam that uses the carrier's stroke to push your existing trigger forward. The selector path therefore needs a separate conventional trigger; the trigger path does not. The next section covers the selector route.

The selector route uses the Nexus Super-Safety Trip (SKU K-2-003) in place of the FRT L3 trip, plus a conventional AR-15 trigger and a forced reset selector. It requires the Nexus AR-style Kuna lower and is not compatible with the OEM Springfield Kuna lower. Its compatibility is written against a class of selectors rather than two named trigger models, which gives it a wider range of working combinations than the FRT trip.

Nexus lists three selector options: any standard Super-Safety, the AS Designs selector, and the Atrius Super-Selektor. The AS Designs Arc-Fire V2 is the one to run. It ships with its own pre-cut trigger cut to Hoffman Tactical specs, and it is ambidextrous out of the box with M2 tool steel DLC-coated wear components. Nexus notes it has done less in-house testing on the AS Designs selector than on Atrius and is going on customer reports.

Budget for a fire control group on the Arc-Fire path, because this is where a Kuna build differs from an AR build. The Arc-Fire kit supplies the trigger only and expects you to reuse the hammer, disconnector, and springs already in your lower. A Nexus Kuna lower ships empty, and the Kuna's factory fire control group is proprietary and stays with the original lower, so there is nothing to reuse. You need an AR-15 hammer, disconnector, hammer spring, trigger spring, and disconnector spring on top of the kit, whether bought loose or as a lower parts kit. The Rare Breed and Disruptor paths avoid this entirely because both are self-contained cassettes.

On a standard Super-Safety, Nexus recommends a centering block spacer to keep the selector centered, and notes it is sold with the Super-Safety rather than by Nexus. That is a Super-Safety part; the Arc-Fire and Atrius are different designs and Nexus does not call for one on either. Trigger pairing then splits by selector, and the two are opposites. On the Atrius path, Nexus tested the Super-Selektor with a drop-in RISE RA-140 and states the RA-140 is the only trigger it recommends for Atrius. On the Arc-Fire path you use the pre-cut trigger in the kit: AS Designs lists the Arc-Fire as incompatible with the RA-140, along with the LaRue MBT-2S and CMC Single Stage. Do not carry the RA-140 recommendation across from one selector to the other.

Nexus does not sell Super-Safeties, the Atrius Super-Selektor, or any FRT. A complete selector build is the lower, the trip, a selector, a conventional AR-15 trigger, and an AR-15 grip, sourced separately. For selector operation, break-in, and dry-fire practice, see the Super-Safety guide.

Confirm the gun resets across a full magazine before running it hard. Three forced reset failures look similar and have different causes: a trigger that fires once or twice then goes dead with no hammer fall is a reset problem, a hammer that falls on a shallow primer dent is a light strike, and a gun that fires before full lockup is hammer follow. Diagnose which one you have before changing parts.

The HB Industries Kuna Short Stroke Buffer Kit ($40) is the standard reliability upgrade on a factory Kuna, closing the gap that lets spent cases drop behind the buffer, and it includes a +20 percent recoil spring. HB Industries warrants it only in the factory configuration and does not guarantee it will cycle reliably behind an AR-15 fire control group, which is why some builders keep the factory buffer on a converted gun. It is a recoil part rather than a reset-timing part.

There is also a documented conflict between that buffer kit and a trip build. Kuna owners running an Atrius selector on the Nexus lower and Nexus Super-Safety trip have reported the trip mechanism striking the short-stroke kit's enlarged buffer and preventing full reset. Nexus said it would test the combination and has not published a result. Run the factory buffer on a trip build unless you are prepared to diagnose that interaction yourself.