M4E1 5.56 Assem Upper Govt 14.5'' Bbl Mid-Length Threaded Blk

The Aero Precision M4E1 5.56 assembled upper receiver with a 14.5" mid-length barrel is a complete, ready-to-run upper for shooters building a compact rifle or upgrading an existing platform. This government-profile upper arrives fully assembled—barrel, bolt carrier group pocket machined and ready, gas system installed, and forward assist in place. The 14.5" mid-length configuration balances velocity, handling, and capacity in a shorter package ideal for tactical work, home defense, or confined spaces where a standard 16" rifle doesn't fit the mission.

The barrel itself is built from 4150 CrMoV steel with a QPQ finish for corrosion resistance, chambered in 5.56 NATO with a 1/7 twist rate that stabilizes a wide range of ammunition weights. The mid-length gas system keeps recoil manageable and pairs well with standard-profile accessories. A standard A2 flash hider comes installed; note that the muzzle device is not pinned or welded, so you'll need to pin and weld it to the barrel to achieve the legal 16" overall length per ATF rifle regulations if that's your build plan. The upper features M4 feed ramps and a low-profile gas block, and comes anodized black.

The included ATLAS R-ONE handguard is Aero's slim, lightweight rail system with M-LOK slots and a full-length top Picatinny rail for optics or accessories. Quick-disconnect sling mounts are positioned at 3, 6, and 9 o'clock. This upper does not include a bolt carrier group or charging handle—source those separately to complete the build.

Key Features

• Government profile upper receiver with enhanced forging and billet-like appearance
• 14.5" mid-length 4150 CrMoV QPQ barrel in 5.56 NATO, 1/7 twist
• M4 feed ramps and .750 gas block journal for standard AR components
• ATLAS R-ONE M-LOK handguard with full top Picatinny rail and QD sling sockets
• Fully assembled and ready to mount on a lower receiver
• Laser-engraved T-marks and .250 takedown pin holes for compatibility
• Low-profile gas block compatible design
• Port door and forward assist installed

Specifications

Frequently Asked Questions

Is this upper receiver compatible with my AR15 lower?

Yes. This M4E1 upper is mil-spec compatible with any standard AR15 lower receiver. The .250 takedown pin holes and M4-profiled feed ramps ensure proper fitment with both commercial and military-pattern lowers.

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

This assembled upper includes the receiver, barrel, gas system, handguard, and accessories. You'll need to source a bolt carrier group (BCG) and charging handle separately, as well as a lower receiver, magazine, and buffer tube assembly to complete a working rifle.

Do I need to pin and weld the flash hider for a legal rifle build?

If you're building a rifle with a 14.5" barrel, yes—per ATF regulations, the muzzle device must be pinned and welded to the barrel to achieve the minimum 16" overall length required for an unregistered rifle. The flash hider on this upper is not pre-pinned or welded, so plan on having a gunsmith complete that work.

What's the difference between the mid-length gas system and other gas lengths?

The mid-length system on this barrel is optimized for 14.5" to 16" barrels and provides a balance between dwell time and recoil impulse. It cycles reliably with standard 5.56 loads and works well with both heavier and lighter-for-caliber ammunition.