BAU-556 Upper Receiver Assembly


The BAU-556 Upper Receiver is the best way to upgrade your existing AR-15 with lightweight, high-performance components by Brigand Arms and our partners. From the lapped receiver, to the ultralight handguard, to the balanced and adjustable competition-grade gas system, every part of this upper receiver ticks the performance up a level. Easy to install without a single tool, the BAU-556 can upgrade your existing M4-style rifle or serve as a lightweight and dependable platform to kick-start your custom build. Most owners find that this Upper Assembly reduces the weight of their rifle by 1.1 to 2.1 lbs. But lightweight alone can lead to jarring recoil and ringing – our adjustable gas system and integrated muzzle brake provide the perfect balance so the rifle has minimal recoil and faster cycling, while providing a firm and well-damped firing sequence.


What makes this the ultimate AR-15 upgrade? Besides tool-free installation and natural style, every piece of the upper was selected by our design team and assembled to bring you out-of-the-box lightweight performance:


ar15 upper receiver assembly with carbon fiber handguard highlighted in red

Our NOAX Carbon Fiber Handguard is optimized to provide high strength and ultralight weight. Aerospace-grade American carbon and machine-controlled interlacing contributes to their unparalleled strength-to-weight ratio, durability, and heat rejection. M-LOK and PIC accessories easily clip into the lattice at any orientation using Brigand Arms adapter plates. (Some of our favorite handguard options are listed, or you can put the ‘no handguard’ option in your cart along with any of our handguards up to 12″ models, and we will install it for free.)


ar15 lightweight upper receiver assembly with faxon barrel highlighted

The Faxon Pencil Barrel is an excellent balance of performance and weight. The integrated brake (compensator) is counted towards the barrel length, so it reduces weight while remaining legal across the USA (The ATF requires rifles to have a 16” barrel even though military M4 rifles are only 14.5”).  The compensator features a three-slot brake that cuts recoil and reduces barrel deflection during repeated firing. Beyond low weight, the button rifling and an 11-degree recessed target crown provide high accuracy.


lightweight ar15 upper receiver with bolt carrier group highlighted

Iron City Rifle Works BCG redefines the limit of lightweight, low-friction bolt carriers. This Gold Competition / Low-Mass G3 Bolt Carrier Group (BCG) is machined from 8620 tool steel, coated using a proprietary PVD that gets slicker over time, and features their heavy-duty Tac-Driver bolt. The BCG is light enough to reduce the overall weight of a AR build, and the balanced mass of the entire assembly reduces the felt recoil and improves the cycle rate of any AR-style firearm.


ar15 lightweight upper receiver build with upper reciever highlighted

The 7075-T6 aluminum upper receiver is forged then re-machined to mil-spec M16/M4 specifications and hardcoat anodized. It features M4 feed-ramps for reliable cycling. Brigand Arms also laps the barrel nut surface before installing barrels, to maintain accuracy at any temperature. Removing the forward-assist provides a sleek profile and reduces weight. Laser etched T-marks on top help align quick-release sights or scopes.


ar15 upper receiver assembly with gas block highlighted

The Aero Precision Adjustable Gas Block allows you to finely tune your gas system. This is particularly useful for managing excess gas to reduce blow-back and mitigate recoil when using a lightweight Bolt Carrier Group. Adjustment screw is held in place by a spring and detent, providing repeatable audible “clicks” upon adjustment removing the need for secondary set screws that can come loose under recoil. The gas block is adjusted to the optimal settings for 55 grain 5.56 ammunition with a mil-spec carbine buffer tube and weight. The BAU-556 includes straightforward instructions for tuning the system to your favorite ammunition.


ar15 complete upper receiver assembly with dust cover and charge handle highlighted

The Strike Latchless Charging Handle offers forged 7075 construction and efficient side gas venting. The heat-resistant polymer iso-latch adds a larger grip surface for dexterous and ergonomic handling. It’s especially helpful for charging a scoped rifle where mil-spec charge handles are notoriously hard to reach. The upgraded Ultimate Dust Cover contains an over-molded rubberized soft sealing lip to prevent debris and water from entering when the cover is closed. Polymer components reduce noise when charging the rifle.


  • Caliber: 5.56 NATO
  • Weight: 2lbs 11oz (1220g) (with Iron City BCG and 12″ NOAX Edge Handguard)
  • Overall Length: 23.74 inches

We offer the BAU-556 Upper Receiver Assembly with these options:

  • BCG & Handguard is exactly what you see in the images – a complete system with a 12″ NOAX Edge handguard and lightweight bolt-carrier-group. This is the best option if you like what you see, and want this to work out-of-the-box
  • No Handguard gives you the ability to customize your BAU-556 with any handguard from our website. Simply add this product to your cart along with another handguard from our site, and we will install that guard before shipping. (Please note that 15″ handguards are longer than this barrel and should not be used)
  • Stripped (No BCG) is the same as BCG & Handguard but does not include the BCG. This option is best if you already have a 5.56 NATO Bolt carrier group, including re-using the bolt from your existing build. (As with any rifle, bolts/barrels should be checked for headspace after assembly)
  • Stripped (No BCG or Handguard) is the same as No Handguard but does not include the BCG. This option is best if you already have a 5.56 NATO Bolt carrier group, including re-using the bolt from your existing build. (As with any rifle, bolts/barrels should be checked for headspace after assembly)


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