BRN-180 | TRIAD Bullpup Chassis

Bring a compact bullpup layout to your Brownells BRN-180 with the A3 Industries TRIAD Modular Bullpup Chassis. The TRIAD houses a standard mil-spec AR lower receiver and pairs it with a bufferless upper, giving you the versatility of the AR platform in a bullpup package. This complete kit includes the TRIAD chassis and the installation kit made specifically for the BRN-180.

Why a bullpup?

A bullpup shortens your firearm's overall length without shortening the barrel. With a 16" barrel, the TRIAD is about 9.75" shorter overall than a conventional AR set to the same length of pull, making it far more maneuverable in tight spaces. Moving the bolt and barrel rearward also shifts the center of mass behind your grip, for better balance and control, and the shorter distance between the action and butt plate sends recoil more directly into your shoulder with less muzzle rise.

A better bullpup trigger

The TRIAD is designed to avoid the typical "bullpup trigger" feel. Instead of a sliding trigger bar, it uses a traditional pivoting front trigger and a bell crank that converts the pull of the front trigger into a push on the rear trigger. The result is a natural-feeling trigger with a short, rigid rear linkage that weighs just 0.8 oz. Oil-embedded bronze bushings and ±0.001" tolerances at every pivot point provide smooth, virtually frictionless movement. A light linkage return spring adds about one pound to your trigger's pull weight.

Pistol or rifle configuration

• Rifle: includes a standard compact butt plate and a 1.5" extended butt plate, a cheek rest, and two steel QD sockets.
• Pistol: includes a featureless receiver end cap with no cheek rest and a rear-facing sling socket (three QD sockets total), designed to prevent shouldering. If you later register your firearm as an SBR, a butt plate upgrade kit is available.

Brownells BRN-180 compatibility

The TRIAD works with BRN-180 uppers in 5.56x45 (16" and 18.5" barrels), 300 BLK (10" barrel), and 7.62x39 (10.5" and 16" barrels). The BRN-180 is the only compatible upper that runs 7.62x39 in the TRIAD, since it uses AR-style 7.62x39 magazines. If you own multiple BRN-180 uppers, you can swap between them on the same TRIAD with no changes.

What's included

• TRIAD chassis body with front trigger housing assembly (front trigger and selector installed, adjusted, and tested)
• Brownells BRN-180 installation kit
• All internal and assembly components (no AR lower parts kit needed)
• Assembly wrenches and instructions

Additional components required

• Brownells BRN-180 upper receiver
• Standard forged mil-spec AR lower receiver
• AR bolt catch with spring, pin, and plunger
• Single-stage, flat-faced AR trigger
• 3/8" drive ratchet for assembly

Lower receiver compatibility

The TRIAD requires a standard forged mil-spec AR lower receiver. Billet lowers and lowers with integrated trigger guards are not compatible. Verified compatible lowers include PSA, Colt, Anderson, Aero Precision (without integrated trigger guard), and Spikes Tactical (without integrated trigger guard).

Trigger compatibility

Any single-stage trigger with a flat (not curved) face should work. We recommend a short-reset cassette trigger for best performance. CMC and Radian Vertex triggers are not compatible due to their size.

Assembly

Assembly is straightforward and takes about 30 minutes. Remove the left chassis half, install your lower with the trigger and bolt catch already in place, reinstall the chassis half, and adjust the trigger actuator. Then install the end cap or butt plate and mount the M-LOK front takedown lug on your upper.

Construction and included components

• 6061 aluminum front trigger housing
• 7075 aluminum front trigger
• Three 4140 steel anti-walk trigger pins
• A3 ambidextrous selector with detent and spring
• QPQ-coated 17-4 stainless steel front trigger linkage
• 7075 aluminum transfer bar and rear linkage with 841 oil-embedded bronze bushings
• Acetal (Delrin) guide and trigger actuator
• Injection-molded, 30% glass-reinforced Zytel main chassis housing
• 4140 steel and 7075 aluminum threaded chassis inserts
• 4140 steel QD sockets: two on rifle configurations, three on pistol configurations
• 4140 steel magazine release
• 7075 aluminum extended right-side magazine release button and left-side lever
• Three 4140 steel ball-detent takedown pins
• A3 Xeno-patterned, reduced-angle polymer grip

Overall length and legal compliance

The TRIAD is very compact. A 16" barreled upper produces an overall length of just 24.75" with the standard butt plate, which is under the 26" minimum for a rifle. To reach 26" or more in rifle configuration, use one of the following:

• A 16" barrel with the 1.5" extended butt plate (included with rifle kits)
• An 18" or longer barrel with the standard butt plate, such as the 18.5" BRN-180
• A 16" barrel with a pinned and welded muzzle device at least 1.5" long

BRN-180 uppers with 10" or 10.5" barrels must be built in pistol configuration or registered as a short-barreled rifle (SBR) with the ATF before being assembled as a rifle. You are responsible for ensuring your configuration complies with all federal, state, and local laws, including the National Firearms Act (NFA) and Gun Control Act (GCA).

The TRIAD is a chassis, not a lower receiver. It is an unregulated accessory and ships directly to your door with no FFL transfer required.

Important: Review the TRIAD Info FAQ before ordering for full configuration details, compatibility requirements, and specifications.