Building an 8.6 Blackout AR
Introduction
In this build guide, Caleb from Brownells takes on the challenge of assembling a large frame AR308 upper chambered in 8.6 Blackout. Often described as the AR308’s version of the 300 Blackout, the 8.6 Blackout is based on a shortened and necked-up 6.5 Creedmoor case, pushing a .338 projectile. The result is a cartridge optimized for subsonic performance and suppressed shooting while also serving as an effective hunting round, especially for hogs and medium game.
Lower Receiver Setup and Parts Used
The lower receiver for this build is already complete, featuring an Aero Precision AR308 lower paired with a Die Free Company grip and plug. For stability and comfort, a B5 Systems precision stock in multicam is installed, designed with a shorter cheek piece for proper clearance on AR308 platforms.
Aero Precision Upper and Handguard Selection
To match the lower, an Aero Precision AR308 upper receiver is used. For the handguard, Caleb selects the Midwest Industries Combat HD high-profile handguard, built tough and designed for AR308 rifles. Known for their strength and ease of installation, Midwest Industries handguards are a go-to option for both AR-15 and AR308 platforms.
Faxon 8.6 Blackout Barrel and Dead Air Muzzle Device
The build uses a Faxon Firearms 16-inch 8.6 Blackout barrel with a 1:3 twist rate, standard for stabilizing heavy subsonic bullets. Uniquely, the barrel is threaded 18x1.5 metric, but Dead Air offers compatible muzzle devices. For this project, Caleb uses a Dead Air muzzle brake to pair with a suppressor-ready setup.
Suppressor Choice and Charging Handle
Suppressor selection is crucial for the 8.6 Blackout, and Caleb chooses the Dead Air Nomad 33, designed for .338 caliber use. This suppressor will also work on future 338 ARC builds. For charging, a Badger Ordnance ambidextrous charging handle with wide handles is installed for easier manipulation under optics.
Strike Industries Arca Rail for Tripod Mounting
To enhance versatility, a Strike Industries Arca rail is mounted under the handguard. This feature allows the rifle to be quickly secured to a tripod for stability during precision shooting or hunting applications.
Barrel Installation and Barrel Nut Torque
The barrel extension is greased and installed into the upper receiver. The Midwest Industries barrel nut is torqued to spec, creating a secure and reliable fit. Unlike gas-operated AR-15s, the 8.6 Blackout AR308 build does not require aligning for a gas tube, which simplifies the process.
Gas Block and Gas Tube Installation
A standard low-profile gas block is installed along with the gas tube. While this build starts with a fixed gas block, an adjustable version could be added later if needed to fine-tune suppressed performance. Alignment is checked using the handguard channel for precision fit.
Handguard Fit and Alignment
The Midwest Industries Combat HD handguard slides into place, secured with a cross-bolt and torqued screws. Anti-rotation features ensure the handguard stays locked in position under heavy use.
Muzzle Device Timing with Shims
The Dead Air muzzle brake is installed using provided shims for proper alignment. Unlike crush washers, shims provide consistent spacing and prevent uneven torque that could misalign the suppressor. Once installed, the brake is timed so its large ports are perfectly horizontal.
Optic Installation (Sig Tango MSR)
For optics, Caleb uses a Sig Sauer Tango MSR, mounted securely to the upper receiver rail. The optic is positioned slightly forward for proper eye relief and stability, then snugged down for reliability under recoil.
Suppressor Fitment and Mounting System
The Dead Air Nomad 33 is attached using the Zeno mount system, which uses left-hand threads and a taper lock. This design prevents carbon lock and ensures the brake does not back out when the suppressor is removed after extended use.
Installing Arca Rail on Handguard
The Strike Industries Arca rail is mounted at the rear of the handguard for balance, allowing the rifle to lock directly into a tripod. Extended M-LOK nuts make installation easier, even in tight spots near the gas block.
Charging Handle and BCG Lubrication
A Brownells AR308 bolt carrier group is lubricated and installed along with the Badger Ordnance charging handle. Proper lubrication ensures smooth cycling and reliability, especially important for suppressed setups.
Upper and Lower Assembly
The Aero Precision AR308 upper and lower are pinned together, completing the large frame 8.6 Blackout build. The suppressor is reinstalled, and the rifle is visually inspected for fit and finish.
Suppressor Reinstall and Final Build Look
With the suppressor back in place, the completed build showcases a balanced mix of black and flat dark earth components, accented by a multicam stock. Caleb notes that future cosmetic adjustments may be made, but the rifle is already a powerful and capable platform.
Next Steps: Handloading 8.6 Blackout and Range Test
The next stage of this project will involve handloading 8.6 Blackout rounds using Hornady bullets and dies, followed by range testing to evaluate accuracy, recoil, and suppressor performance. This cartridge is ideal for hunters, suppressor enthusiasts, and anyone seeking something different in a large frame AR308 platform.
