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

The Aero Precision M4E1 300 Blackout assembled upper receiver pairs a precision-forged upper with a 16-inch .300 BLK barrel and the company's slim-profile ATLAS R-ONE handguard—a complete, ready-to-run platform for hunters and shooters building a subsonic or supersonic 300 Blackout rifle. This upper comes fully assembled with a low-profile gas block, threaded muzzle, and all the internal goodies installed, so you're ready to mate it to a lower receiver, bolt carrier group, and charging handle to get on target.

The 16-inch barrel is built from 4150 Chrome Moly Vanadium steel with a 1-in-7 twist, optimized for standard and heavier 300 Blackout loads. The muzzle is threaded 5/8x24 and finished with an A2 flash hider (not pinned or welded, so you can swap it for a suppressor or muzzle brake if needed). The QPQ corrosion-resistant finish protects the barrel inside and out. The gas system is set to pistol length, pairing with the low-profile gas block for a clean look under the handguard.

The M4E1 upper receiver itself features an enhanced forging that delivers a billet aesthetic without the weight penalty, plus refined picatinny rails that mesh seamlessly with Aero's Extended Series handguards and accessories. It includes M4 feedramps, port door, forward assist, and laser-etched T-marks. The ATLAS R-ONE handguard spans 15 inches, keeping weight minimal while offering full top rail real estate and quick-detach sling mounts at 3, 6, and 9 o'clock. Its proprietary ATLAS mounting system is rock-solid and maintains a slim 1.3-inch inner diameter—compatible with standard mil-spec AR barrels and low-profile gas blocks.

This is an assembled upper—no BCG or charging handle included—so you'll need those components to complete your build. If you run suppressors or subsonic ammo, the 300 Blackout platform in a 16-inch format strikes a practical balance between velocity and package size.

Key Features

• Complete assembled upper receiver ready to install on a lower
• 16-inch .300 Blackout barrel with 1-in-7 twist, optimized for subsonic and standard rounds
• 5/8x24 threaded muzzle with A2 flash hider (removable, not pinned/welded)
• Enhanced-forged M4E1 upper receiver with billet-style look and refined picatinny profile
• ATLAS R-ONE 15-inch handguard with full top rail and 3/6/9 o'clock QD sling mounts
• Low-profile gas block paired with pistol-length gas system
• M4 feedramps, port door, and forward assist pre-installed
• 4150 Chrome Moly Vanadium barrel with QPQ corrosion-resistant finish throughout
• Slim, lightweight design without compromising attachment options

Specifications

Frequently Asked Questions

What do I need to complete this upper into a working rifle?

This is an assembled upper receiver only. You'll need a lower receiver, a bolt carrier group (BCG), and a charging handle to finish the build. Once those are paired with this upper, you'll have a complete, functional rifle.

Is the muzzle brake pinned and welded?

No. The A2 flash hider that ships with this upper is removable, not pinned or welded. You can swap it out for a suppressor or other muzzle device that fits the 5/8x24 threading if desired.

What handguard comes with this upper?

The ATLAS R-ONE handguard (15 inches long) is included. It features a full top picatinny rail for optics and accessories, plus quick-detach sling mounts at the 3, 6, and 9 o'clock positions.

Is this upper compatible with my existing AR15 lower and bolt carrier group?

Yes, this M4E1 upper is compatible with standard mil-spec AR15 lower receivers and bolt carrier groups. You'll need to source your own BCG and charging handle to complete the build.