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