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 our Enhanced and ATLAS Series Handguards- M4 Feedramps- .250 takedown pin holes- Laser engraved T-marks