USAF GUU-5/P Style Upper, 223 Rem/556 NATO, 14.5" TNTE Chrome Lined Barrel (16" OAL Pinned and Welded), 1:7 Twist, Black Finish, Extended A2 Flash Hider pinned and welded.
‘Mixmaster’: The U.S. Air Force’s GUU-5/P – Small Arms Defense Journal (sadefensejournal.com)
ABOVE: An airman fires a later model GUU-5/P with the M4-type barrel based on an early “slab-side” lower reciever (Air Force)
In January 1991, the U.S. Air Force’s 24th Security Police Squadron at Howard Air Force Base in Panama found itself in a predicament. The unit’s explosive ordnance disposal elements had a shortage of guns. For more than three years, the team’s request for four carbines had been on back order.
So the squadron sent a request up the chain to the Warner Robins Air Logistics Complex at Robins Air Force Base in Georgia asking for permission to use their own money to do the work themselves. Armorers at Howard would turn three M16s – with serials in the 300,000 range, dating them to the 1960s – into new carbines, called GUU-5/Ps.