The Aero Precision M4E1 Threaded Receiver Assembly Set includes the M4E1 Lower Receiver  and the M4E1 Threaded Assembled Upper Receiver.

M4E1 Lower Receiver Features

• Threaded Bolt Catch Roll Pin – Allows for simple installation of the bolt catch and virtually eliminates the chance to damage the finish during installation (pin included).
• Integrated Trigger Guard – Eliminates the possibility of breaking the trigger guard tabs by integrating the trigger guard into the lower, creating a stronger more rigid platform to build upon.
• Upper Tension Screw – Allows users to fine tune the fit of the upper and lower receiver using a nylon tipped tensioning set screw inserted in the grip tang of the lower receiver. This provides a tight fit with any standard AR15 upper receiver.
• Threaded Takedown Pin Detent Recess – Allows user to easily install the Takedown Pin detent and spring with the use of a 4-40 set screw (no more launching detents across the room).
• Increased Magwell Flare – Increased the flare of the magwell to aid in quick and efficient magazine changes.

M4E1 Threaded Upper Receiver Features

• New enhanced forging that gives the upper receiver a “billet look”
• New picatinny profile that blends seamlessly with Aero Precision’s Enhanced and ATLAS Series Handguards
• M4 Feedramps
• .250 takedown pin holes
• Laser engraved T-marks