M4E1 300Blk Assm Upper Gov 8''Bbl Pistol-Length Threaded Blk

The Aero Precision M4E1 .300 Blackout assembled upper with 8-inch barrel is a complete, ready-to-run upper receiver built for shooters who prioritize compact size and lightweight handling. Chambered in .300 Blackout with a 1:7 twist rate, this upper pairs a 4150 CMV barrel—finished with a QPQ corrosion-resistant coating—with Aero's ATLAS S-ONE handguard system, a slim-profile M-LOK rail designed to shed weight without sacrificing strength or utility. The 8-inch barrel length and pistol-length gas system make this an excellent choice for short-range shooting, suppressed fire, or compact builds where maneuverability matters. This is a complete upper receiver assembly ready to mate with your lower—you'll just need to source a bolt carrier group and charging handle separately.

The M4E1 enhanced forging gives the upper a refined appearance while maintaining the durability Aero Precision is known for. The ATLAS S-ONE handguard strips weight through intelligent design: it eliminates the center of the top rail, keeps the profile slim at 1.3 inches inside diameter, and still delivers front and rear picatinny rails plus M-LOK mounting along the sides. Quick-disconnect sling points at 3, 6, and 9 o'clock give you flexibility for how you carry and deploy. A low-profile gas block and pistol-length gas tube come installed, and the muzzle features a 5/8x24 thread for suppressors or brakes—though the included brake is not pinned or welded, so you can swap it as needed.

Built from the ground up to be a cohesive system rather than a collection of mismatched parts, the M4E1 upper handles .300 Blackout's subsonic potential and delivers the compact footprint modern shooters expect. The 8-inch barrel weighs just 18 ounces and has been HP and MPI tested to verify integrity. If you're building a short-range workhorse or an integrally suppressed AR pistol, this upper gives you a proven platform without guesswork.

Key Features

• Complete assembled upper receiver—install on any mil-spec AR15 lower and add BCG/charging handle
• 8-inch .300 Blackout barrel, 1:7 twist, 4150 CMV with QPQ finish for corrosion resistance
• ATLAS S-ONE lightweight handguard with front/rear picatinny and full M-LOK coverage
• Low-profile gas block and pistol-length gas tube included and installed
• 5/8x24 threaded muzzle ready for suppressors, muzzle brakes, or other devices
• M4E1 enhanced upper forging with M4 feedramps and laser T-marks
• QD sling attachment points at 3, 6, and 9 o'clock positions
• Port door and forward assist pre-installed

Specifications

Frequently Asked Questions

What do I need to complete this upper receiver into a functioning rifle or pistol?

This is a complete upper assembly—it includes the receiver, barrel, handguard, and gas system. To finish the build, you'll need to add a bolt carrier group and charging handle. You'll also need a lower receiver, magazine, and fire control group to create a complete firearm. Check local and federal laws before assembling.

Is the muzzle brake installed, and can I replace it with a suppressor?

Yes, the muzzle brake comes installed on the 8-inch barrel. It is not pinned or welded, so you can remove it and install a suppressor, flash hider, or other muzzle device with a 5/8x24 female thread. If you plan to use a suppressor, verify it is rated for .300 Blackout and that your build complies with federal and local regulations.

Will this upper work with my existing AR15 lower receiver?

Yes, the M4E1 upper is compatible with mil-spec AR15 lower receivers. The .250 takedown pin holes and standard M4 feedramps ensure drop-in fitment with any standard lower platform.

What is the ATLAS S-ONE handguard, and why is it different from other Aero Precision rails?

The ATLAS S-ONE is Aero Precision's lightweight handguard, designed with weight savings as a priority. It eliminates the center of the top rail (which many shooters don't use) to reduce overall mass while maintaining front and rear picatinny rails and full M-LOK support on the sides. It includes QD sling sockets at multiple points and is compatible with low-profile gas blocks, making it ideal for compact and lightweight builds.