M4E1 300Blk Assemble Upper Gov 16''Bbl Pistol-Length Thrd Blk

The Aero Precision M4E1 300 Blackout complete upper with 16" barrel is a proven platform for building a dedicated short-mag rifle or pistol-caliber upper. This assembled upper pairs Aero's M4E1 forged receiver with a CMV barrel, low-profile gas block, and their new ATLAS S-ONE handguard—a lightweight, slim-profile rail system that doesn't sacrifice strength or versatility. The 16" 300 Blackout barrel runs a pistol-length gas system, making it ideal for subsonic ammunition, suppressor-ready builds, or precision shooting where .300 BLK's ballistics shine. Whether you're building a specialized hunting upper, a range gun, or experimenting with 300 Blackout's unique capabilities, this upper gives you a quality foundation from one of the industry's most respected manufacturers.

The M4E1 forged receiver features Aero's enhanced design with a billet-like appearance and updated picatinny profile that plays well with modern handguards. M4 feedramps ensure reliable feeding, and you get standard M4 takedown pin holes, laser-engraved T-marks, and a pre-installed port door and forward assist. The 16" CMV barrel is HP and MPI tested, threaded 5/8x24 for suppressor mounting, and finished with QPQ corrosion-resistant treatment inside and out—important for reliable function and longevity in the field.

The ATLAS S-ONE handguard is where this upper distinguishes itself. It's built around Aero's proprietary ATLAS attachment system and comes with a 12" M-LOK rail segment, delivering mounting real estate without unnecessary bulk. The design eliminates the center of the top rail to shed weight, includes front and rear picatinny sections, and features quick-disconnect sling socket positions at 3, 6, and 9 o'clock. Everything ships ready to drop onto a compatible lower—just add a bolt carrier group and charging handle to complete the rifle.

Key Features

• M4E1 forged receiver with enhanced design and billet-like profile
• 16" .300 Blackout CMV barrel, 1:7 twist, threaded 5/8x24
• Pistol-length gas system with low-profile gas block
• ATLAS S-ONE 12" M-LOK handguard with weight-optimized design
• Quick-disconnect sling mounts at multiple positions
• M4 feedramps and standard takedown pin holes
• QPQ corrosion-resistant barrel finish
• HP and MPI tested barrel
• Includes port door and forward assist

Specifications

Frequently Asked Questions

What's included and what do I still need to complete a rifle?

This is an assembled upper receiver only. You'll need to source a lower receiver, bolt carrier group, and charging handle to complete a functioning rifle. The upper ships with the receiver, barrel, gas block, gas tube, and handguard already assembled and ready to mount.

Is the muzzle brake pinned and welded?

No. The muzzle device that ships with this upper is not pinned or welded, so you can swap it for a suppressor, flash hider, or other 5/8x24 muzzle device as needed.

Can I use this upper on a standard AR-15 lower?

Yes. The M4E1 upper is compatible with mil-spec AR-15 lower receivers and maintains standard .250 takedown pin holes for reliable mounting.

Why choose .300 Blackout over 5.56 or other rifle calibers?

300 Blackout excels with subsonic ammunition and suppressed fire, making it popular for hunting medium game and precision applications where lower velocity and hearing-safe ballistics are advantages. The 16" barrel with pistol-length gas system is a practical balance for this cartridge, and the threaded muzzle supports suppressor mounting.