Davidson Defense Breacher Up w/Gauntlet Brace 10.5-inch 1:8 .300 BLK AR-15 FSB Quad Pistol Full Build Kit

Breacher Up w-Gauntlet Arms Brace 10.5-Inch 1-8 .300 BLK AR-15 FSB Quad-Rail Pistol Full Build Kit

This is an AR-15 pistol full build kit chambered in .300 AAC Blackout, centered on a 10.5-inch government profile 4150 CrMoV steel barrel with a 1-8 twist rate and a pistol-length gas system. The upper assembly is built on a 7075-T6 aluminum upper receiver with an anodized finish, M4 feed ramps, a standard forward assist, and a .223 and 5.56 dust cover assembly. The barrel carries an A2 front sight base with a complete sight post, plunger, and spring, and a separate adjustable skeletonized clamp-on gas block at .750 inch positioned under the handguard that drives the pistol-length gas system independently of the front sight base. The 7-inch M4 carbine drop-in quad rail with a Colt-style delta ring assembly covers the forend. A complete M16 profile bolt carrier group and a Mil-Spec M4 charging handle complete the upper side. The lower completion package covers a complete Mil-Spec AR-15 lower parts kit, a heavy-duty pistol buffer tube, a standard carbine buffer at 3 oz., a standard carbine recoil spring, a Mil-Spec AR-15 M16 receiver end plate, and a Mil-Spec AR-15 carbine castle nut. A Gauntlet Arms black polymer stabilizing brace closes out the rear end. Lower receiver not included.

Key Features

• .300 AAC Blackout chambering, 10.5-inch government profile 4150 CrMoV steel barrel, black finish
• 1-8 twist rate, 5-8x24 thread pitch
• Pistol-length gas system with pistol-length gas tube
• Adjustable skeletonized clamp-on gas block, .750 inch, under handguard
• A2 front sight base with sight post, plunger, and spring
• 7075-T6 aluminum upper receiver, anodized, M4 feed ramps
• Standard AR-15 forward assist assembly
• .223 and 5.56 dust cover assembly
• 7-inch M4 carbine drop-in quad rail, black, Colt-style delta ring assembly
• Complete M16 profile bolt carrier group, .223 Rem, 5.56 NATO, .300 BLK, .350 Legend compatible
• Mil-Spec AR-15 M4 charging handle
• Complete Mil-Spec AR-15 lower parts kit
• Heavy-duty pistol buffer tube
• Standard carbine buffer, 3 oz.
• Standard carbine recoil spring
• Mil-Spec AR-15 M16 receiver end plate
• Mil-Spec AR-15 carbine castle nut
• Gauntlet Arms polymer stabilizing brace, black
• GI Birdcage flash hider, 5-8x24 thread pitch

Perfect For

• AR-15 pistol builders who want a classic FSB and quad rail front end on a .300 BLK pistol-length platform
• Shooters pairing this kit with an existing stripped or complete AR-15 lower receiver
• Builders who want a Gauntlet Arms brace and a complete pistol component set with a classic quad rail and front sight base aesthetic

Upper Assembly and Gas System

The 7075-T6 aluminum upper receiver with an anodized finish and M4 feed ramps forms the core of the front end, with a standard forward assist assembly and a .223 and 5.56 dust cover assembly built in. The 10.5-inch government profile 4150 CrMoV steel barrel carries an A2 front sight base with a complete sight post, plunger, and spring at the carbine-length position on the barrel. The A2 front sight base on this build is a sight-only component and does not serve as the gas block. The actual gas system is driven by a separate adjustable skeletonized clamp-on gas block at .750 inch that sits under the handguard at the pistol-length gas port position on the barrel, with a pistol-length gas tube running from the gas block back to the upper receiver. This configuration delivers a true pistol-length gas system while retaining the classic A2 front sight post look that the FSB provides above the handguard line. The 7-inch M4 carbine drop-in quad rail with a Colt-style delta ring assembly covers the barrel and gas block along the forend, providing full Picatinny rail coverage on all four sides for accessory attachment.

Bolt Carrier Group and Charging Handle

The complete M16 profile bolt carrier group is compatible with .223 Remington, 5.56 NATO, .300 Blackout, and .350 Legend, making it a versatile and well-suited choice for this chambering. The Mil-Spec M4 charging handle pairs with the BCG inside the upper receiver to complete the upper interior and finish the front end of the build.

Lower Completion Components

The complete Mil-Spec AR-15 lower parts kit covers the full internal lower component group, including the fire control group, magazine catch, bolt catch, and all associated pins and springs. The heavy-duty pistol buffer tube, standard carbine buffer at 3 oz., and standard carbine recoil spring form the buffer system at the rear of the lower, matched to the pistol platform. The Mil-Spec AR-15 M16 receiver end plate and Mil-Spec AR-15 carbine castle nut lock the buffer tube down cleanly at the rear of the lower receiver. The Gauntlet Arms black polymer stabilizing brace fits over the pistol buffer tube and closes out the rear end of the build.

Muzzle Device

The GI Birdcage flash hider threads onto the 5-8x24 muzzle end of the barrel and closes out the front of this pistol-length upper with a standard proven flash hider pattern compatible with .300 Blackout at the 5-8x24 thread pitch.

Follow safe handling and manufacturer instructions. Ensure compliance with all applicable federal, state, and local laws. Lower receiver not included.