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