Armalite M15A4 upper. You can see in one image it is attached to the original lower. This lower has the roll-stamp from Geneseo Illinois and the Eagle Arms white color logo on the front magwell. The SN is US100***, I verified with the compliance department through Armalite customer service in Bryan, TX that this rifle was manufactured in 2000. You can see it has no bayonet lug and the muzzle brake is pressed onto the barrel (not threaded). During the Clinton assault weapons ban era, certain manufacturers had to omit variations from AR15 rifles to make the weapon compliant. No bayonet lug or flash hider made the rifle "less dangerous". This upper is all factory and is a mid length gas system.