‘Mixmaster’: The U.S. Air Force’s GUU-5/P ‘Mixmaster’: The U.S. Air Force’s GUU-5/P

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.

Line drawing of the parts for the GAU-5/A, GAU-5A/A and GUU-5/P from a 1989 copy of the official technical order. (Air Force)

By the end of the month, Warner Robins gave the go ahead to the plans. The squadron just had to make sure that it changed the National Stock Numbers for the weapons in the logistics database to make sure everything evened out on paper. Otherwise, if something broke, they might get replacement parts for the older rifles instead of what they actually needed. The author obtained these messages and other records related to the GUU-5/P through the Freedom of Information Act.

The 24th was not alone in the desire for more modern weapons. Throughout the 1990s, Air Force units steadily converted older M16-type rifles and carbines into GUU-5/Ps. While they shared a common theme of having collapsible buttstocks and 14.5-inch barrels, the new firearms were often each a bit unique. Online, enthusiasts on message boards like ar15.com dubbed them “frankenguns” or “mixmasters” on account of the various features found from one gun to the next. The Air Force has been working to replace these conversions with new M4 carbines. However, official instructions still called for these guns as recently as 2014.

Born of need and limited resources, the ad hoc the nature of the GUU-5/P is not necessarily surprising. Historically, the Air Force is not widely known for their work on small arms. “No AF led small arms programs,” Col. Patrick Lopardi, then head of the Programs, Requirements and Innovations Division at the Air Force Security Forces Center stated bluntly in one 2009 briefing. Lopradi was explaining the state of the service’s combat arms program to attendees of the National Defense Industry Association’s International Infantry and Joint Services Small Arms Systems Symposium in Las Vegas, Nevada.

Another GUU-5/P with the M4-type barrel, but with a later model lower receiver (Air Force)

However, the Air Force actually has a long and complex history with the “black rifle.” While the U.S. Army had been ambivalent when they first examined the AR-15 in the late 1950s, the fledgling flying branch was soon taken with ArmaLite’s “space age” lightweight aluminum and plastic rifle. In particular, the company’s parent corporation Fairchild actively lobbied through then head of Strategic Air Command and later Chief of Staff of the Air Force, Gen. Curtis LeMay.

On July 4th, 1960, LeMay famously attended a party to celebrate Independence Day and the birthday of former Fairchild president Richard Boutelle. Boutelle and others made sure the general found time to blast some watermelons with an AR-15. Afterwards, LeMay campaigned to replace the Air Force’s aging World War II-era M-2 carbines with the much more modern gun.

Less than two years later, the Air Force became the first service to adopt the weapon. The Army followed suit and requested a variant with additional features – most notably a forward assist mechanism to push the bolt into battery if it didn’t seat correctly – known as the XM16E1. As a result, the Pentagon retroactively renamed the earlier AR-15s as XM16s, though both terms continued to be used to refer to the rifles. The services standardized these weapons as the M16A1 and M16 respectively.

After putting the full length rifles into service, the Air Force was quick to realize the utility of carbine versions of the gun. In the mid-1960s, Airmen guarding flight lines in Vietnam and South Korea, as well as bases in the United States, began receiving the shorter guns. By that point, Colt had taken over production of ArmaLite’s design and was selling them as the Colt Automatic Rifle-15, or CAR-15, family.

An airmen aims a GAU-5/A “submachine gun” during Operation Desert Shield. (Air Force)

The first of these shortened versions for the Air Force came with a 10.5 inch barrel and a massive flash hider that added another 4 inches to the overall length. With no naming convention for small arms, the service put the gun into the aircraft gun category Aeronautical and Support Equipment Type Designation System. This GAU-5/A became the standard issue weapon for Security Police dog handlers and other specialized personnel. The Army had bought a virtually identical version of the gun from Colt called the XM177E1. Like the M16 and M16A1, the only difference between it and the GAU-5/A was the addition of the forward assist. On paper, logisticians sometimes called the Air Force guns XM177s.

This dichotomy continued when both services started buying the newer guns with slightly longer 11.5” barrels. The Army designated its guns with the forward assist as the XM177E2, while the Air Force dubbed its model without the GAU-5A/A. All of the XM177s and GAU-5s were select fire models with semi- and full-automatic fire modes. The guns also came with early versions of the round hand guard seen on later M16-style guns.

The Air Force guns became most famously associated with the raid on the Son Tay prison camp in North Vietnam on Nov. 21, 1970. Nearly 60 Army Special Forces – some armed with GAU-5A/As specially fitted with the Armson Occluded Eye Gunsite, or OEG – along with more than 90 air force pilots and crew took part in the mission, also known as Operation Ivory Coast.

While the plan was to free American prisoners of war, the raiders found the North Vietnamese had moved the captives just prior to the operation. In 2016, Troy Defense announced the planned release of a limited edition semi-automatic clone of the raider carbine for the American civilian market to commemorate the event. Colt followed suit by announcing a similar, fully functional reproduction at the National Rifle Association’s annual meeting.

As defense spending shrank after the end of the Vietnam War, the Air Force had few reasons to spend valuable resources on new small arms. Still, combined with the introduction of the improved M16A2 rifle in the early 1980s, the service’s armories were becoming increasingly cluttered with an increasingly complicated mix of AR-15 pattern firearms. To help consolidate these weapons, the flying branch cooked up the GUU-5/P.

Air Force Combat Controllers with what looks like M4A1 carbines, but may just be another evolution in the GUU-5/P given the old-style stock (Air Force)

Despite the ostensible goal of standardization, these “submachine guns” can hardly be described as fitting a single standard. Over the years, Air Force armorers assembled them using older M16, M16A1, GAU-5/A and GAU-5A/A as the base. While the goal was a carbine with a 14.5-inch barrel and a collapsible stock, the specifications changed as time went on. Regardless of the exact features, only one National Stock Number – 1005-01-042-9820 – exists for GUU-5/Ps.

Initially, the Air Force simply replaced the existing barrels with a 14.5-inch lightweight or “pencil” type with a 1:12-inch rifling, according one Technical Sergeant at Lackland Air Force Base the author spoke with by phone nearly a decade ago. Retrofitted M16 or M16A1s would need new hand guards, barrel extensions, slip rings and butt stock assemblies.

The resulting weapon was akin to M16A1 carbines like the Colt Model 653 favored by U.S. special operations forces during the late 1970s and early 1980s. It is likely that some of the earliest examples were actually 653s straight from the Colt factory. Via FOIA, the author obtained a series of messages between individuals at Warner Robins discussing the potential to convert GUU-5/Ps rather than purchase new production guns. At the time, the unit cost for whatever this specific Colt model in question was, came to just over $550. Air Force officials estimated it would cost just under $315 to rebuild M16s to the same standard.

These early versions are sometimes referred to as GAU-5/Ps, though this seems to be a typo in some Air Force manuals that has entered the lexicon. The messages from the 24th Wing at Howard and from the personnel at Robins show the GUU-5/P nomenclature was well established by the mid-1980s. The scant official references to the GAU-5/P also use the same NSN as the GUU-5/P.

In addition, armorers would often strike the rollmarks on the lower receiver of converted guns with a series of stamped or eletro-penciled “Xs” and then add the new moniker. Sometimes the airmen would grind out the old markings entirely. While there are numerous examples of new GUU-5/P rollmarks, no photographs have surfaced of a GAU-5/P marking. The author obtained a copy of the 1989 version of the Air Force Technical Order for these firearms through FOIA, which shows the early conversion parts, including the pencil barrel. This document only lists GAU-5/A, GAU-5A/A and GUU-5/P.

As the 1:7-inch rifling became increasingly the default standard for 5.56mm weapons across the Pentagon, the Air Force stopped using the pencil barrels. The M16A2 had been the first to feature this tighter twist in order to stabilize the new, longer M856 tracer cartridge. In turn, GUU-5/Ps started getting 14.5-inch barrels with the same profile as these standard infantry rifles, making them similar to the Colt Model 727. After the introduction of the M4 in 1994, that weapon’s distinctive barrel with the cut for fitting the M203 grenade launcher became the default for Air Force conversions.

A civilian GUU-5/P clone built using parts from various sources. (Photo supplied to author)

All of these barrels were mated to the older upper receivers. So while the barrels were continually updated, the guns retained the older “A1” style rear sights that were adjustable for windage only. Converted GAU-5s carried over the two-position aluminum collapsible stocks coated with vinyl-acetate. Lower receivers could be slab-side without any of the later reinforcements above and around the magazine release. They might only have the partial “fence” found on earlier M16s and GAU-5s, too.

As the conversion program continued, the guns did begin to incorporate more and more features found on M16A2 rifles and M4 carbines. This included upper receivers with the newer rear sights with adjustable elevation, pistol grips with the additional finger grip and collapsible, composite material stocks. But the Technical Sergeant at Lackland made it clear that consistency was not as important as actually getting the weapons built to the basic carbine criteria.

How much the conversion process helped in consolidating Air Force weapons lockers is not entirely clear. By the early 2000s, Air Force instructions asking armorers to list what they had on hand, at various times, included slots for almost every member of the M16 rifle and M4 carbine family imaginable, as well as GAU-5s and GUU-5/Ps. As already mentioned, other documents mentioned issuing GUU-5/Ps only two years ago. At this time, most pictures show carbines with flat top style upper receivers with the integral Picatinny accessory rail and the Colt’s more recent reinforced, collapsible butt stocks. However, it is entirely possible some of these are merely the latest iteration of the conversions.

What is clear, is that the Air Force’s “mixmaster” carbines are a unique and interesting part of the Pentagon’s history with the Black Rifle. As new guns finally push the last of these weapons out of service, hopefully various examples of this particular chapter will find their way into museums where they belong.