M4E1 300Blk Assembled Upper 16''Bbl Threaded W/Rm15'' Hg Fde

The Aero Precision M4E1 300 Blackout assembled upper receiver with 16-inch barrel and FDE ATLAS R-ONE handguard gives you a complete, ready-to-run upper that's built for suppressed shooting, close-quarters work, or subsonic hunting. This isn't a stripped receiver—it comes fully assembled with a quality barrel, gas block, and Aero's proven ATLAS R-ONE handguard system, so you're just adding a bolt carrier group and charging handle to complete your rifle.

The 16-inch barrel in .300 Blackout is threaded 5/8x24 and forged from 4150 Chrome Moly Vanadium with a QPQ corrosion-resistant finish. Aero has paired it with a low-profile gas block tuned to the pistol-length gas system, keeping the upper trim and functional. The M4E1 upper itself features the enhanced forging that gives you a billet-like appearance with M4 feedramps, .250 takedown pin holes, and laser-engraved T-marks. Port door and forward assist come installed.

The ATLAS R-ONE handguard is where this upper really shines for versatility. At 1.5 inches outside diameter, it's slim enough for a compact profile yet wide enough for mounting lights, lasers, or other accessories. Full-length top picatinny rail, QD sling sockets at 3, 6, and 9 o'clock, and Aero's durable proprietary ATLAS attachment system make it a solid foundation for any role. The FDE finish pairs well across your setup and matches modern aesthetic preferences.

Note: This upper does not ship with a bolt carrier group or charging handle—you'll need to source those separately. The muzzle brake is threaded on but not pinned or welded, so it's ready for your suppressor or can be swapped if needed.

Key Features

• Complete, factory-assembled upper receiver ready to pair with a lower
• 16-inch .300 Blackout barrel with QPQ finish for corrosion resistance
• 5/8x24 threaded muzzle for suppressor or brake compatibility
• Low-profile .750 gas block tuned to pistol-length gas system
• M4 feedramps and enhanced forging with billet-like appearance
• ATLAS R-ONE handguard with full top picatinny rail and QD sling sockets
• FDE finish throughout
• Port door and forward assist pre-installed

Specifications

Frequently Asked Questions

What's included in this assembled upper and what do I need to add?

This upper arrives fully assembled with the barrel, gas block, M4E1 upper receiver, and ATLAS R-ONE handguard ready to go. You'll need to source a bolt carrier group and charging handle to complete the assembly before you can fire it.

Can I run a suppressor on this .300 Blackout upper?

Yes. The barrel is threaded 5/8x24, which is the standard for suppressors in this caliber. The muzzle brake comes threaded on but is not pinned or welded, so you can easily swap it for a suppressor.

Is the ATLAS R-ONE handguard compatible with my AR-15 lower?

The ATLAS R-ONE handguard is compatible with standard mil-spec AR-15 upper receivers and barrels, so it will fit any standard mil-spec AR-15 lower receiver when you pair this upper with one.

What's the barrel made from and why does that matter?

The barrel is 4150 Chrome Moly Vanadium with a QPQ corrosion-resistant finish inside and out. This combination delivers durability and longevity, especially important for .300 Blackout builds where reliability and consistency matter.