Brownells Brn-4 5.56Mm 10.4'' Med Bbl Complete Upper W/9'' Handguard

The Brownells BRN-4 5.56mm complete upper receiver is a ready-to-run solution for AR-15 builders who want quality components installed and tested from the factory. This 10.4-inch medium-barrel configuration ships with a 9-inch Midwest Industries handguard, muzzle device, gas system, and bolt carrier group already mounted—just attach it to your lower receiver and head to the range. It's built for shooters who value precision engineering without the assembly hassle.

The core of the BRN-4 is a proprietary bolt carrier group finished in nitride, with the carrier forged from a single piece of 8620 steel and the bolt precision-machined from 9310 tool steel. Both components are individually headspaced before and after barrel installation to ensure reliable feeding and extraction. The Faxon-produced barrel is button-rifled with a 1:7 twist, chambered in 5.56x45mm NATO, and available in medium profile—a balance between weight reduction and thermal stability during extended shooting. The medium profile retains enough material to resist point-of-impact shift as the barrel heats during rapid fire.

The upper receiver is machined entirely from billet aluminum and finished with matte black hard-coat anodizing, mirroring the geometry of traditional forged uppers. A piston bushing is installed and secured with threadlocker to prevent wear. The barrel nut engages over an extended surface area—more thread engagement than a standard AR-15—and is torqued three times during assembly to achieve precise specifications. The handguard offers full-length Picatinny rail and abundant M-LOK attachment points for lights, grips, and accessories. Every assembled upper is test-fired before shipment and may show minor evidence of that final quality check.

Brownells recommends pairing this upper with an H1 or H2 buffer and carbine recoil spring, and using newly manufactured 5.56x45mm NATO ammunition for optimal reliability. This is a complete upper ready for an existing lower—no gunsmithing required.

Key Features

• Complete upper assembly ships ready to attach to an AR-15 lower receiver
• Forged 8620 steel bolt carrier with nitride finish and precision-machined 9310 bolt
• Faxon medium-profile barrel, 10.4 inches, 1:7 twist, 5.56x45mm NATO
• Billet aluminum upper receiver with matte black hard-coat anodizing
• Extended barrel nut engagement for rigid barrel mounting
• 9-inch Midwest Industries handguard with full Picatinny rail and M-LOK slots
• Piston bushing installed and secured to minimize aluminum wear
• Forward-vented gas block tuned for this barrel length
• Magpul ejection port cover and mil-spec charging handle included
• Factory test-fired and headspaced before shipment

Specifications

Frequently Asked Questions

Can I attach this upper to my existing AR-15 lower receiver?

Yes. The BRN-4 is a complete upper assembly designed to attach directly to a standard AR-15 lower receiver. No additional barrel installation or gas system assembly is required.

What recoil buffer and spring should I use?

Brownells recommends an H1 or H2 recoil buffer paired with a carbine recoil spring for optimal operation with this upper.

Is this upper test-fired before shipping?

Yes. All BRN-4 upper assemblies are test-fired before final packaging. You may notice minor evidence of firing, such as brass marks on the shell deflector or slight fouling inside the receiver.

What ammunition should I use?

Brownells recommends newly manufactured, quality 5.56x45mm NATO ammunition for optimal operation and reliability.