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The Bushmaster "Armpistol" (or "Arm Pistol") is rooted in a United States Air Force (USAF) trials program seeking a "Personal Defense Weapon" (PDW) for standard issuance to downed airmen in a combat zone. At this point in American history, the Vietnam War (1955-1975) raged in Southeast Asia so there grew a concerted effort to arm such personnel in the event of their aircraft being shot down Behind Enemy Lines. The program produced prototypes - these carrying the designation of "IMP-221" (IMP = "Individual Multi-Purpose weapon") and known in the USAF inventory as the "GUU-4/P" - but was ultimately not furthered nor adopted. The gun was eventually brought to the civilian market by Gwinn Firearms / Bushmaster before its story ended in the late-1980s.

The original USAF specification of the late-1960s called for a weapon to feature an overall length no more than 13 inches, weighing a handy 1.5 lb or less, and able to reach out to targets at 100 meters - in essence a very portable sidearm with greater capability than a standard service pistol though not quite as bulky and complex as a battle rifle / assault rifle. The design was arranged in a "bullpup" fashion meaning the internal action and feeding mechanism were set aft of the trigger / pistol grip section. This arrangement allowed gun-makers to fashion infantry-level solutions shorter in overall length than typical long-gun counterparts where the feed and action are set ahead of the trigger / pistol grip. For the IMP-221 this proved something of a necessity to stay within the restrictions imposed by the USAF requirements. The barrel assembly measured 11 inches long and overall length became 20.6 inches for the completed product. Weight reached 6.25lb.

Beyond its relatively compact dimensions, the gun was fully ambidextrous from the get-go and the rear section of the receiver was designed to "pivot", or twist, left or right apart from the forward. This feature allowed the shooter to "rest" the gun along either forearm as no conventional shoulder support / stock component for this short-barreled weapon could be fitted. The gun was chambered for .221 "Fireball" (5.56x36mm "Wildcat"), a slightly shorter cartridge when compared tothe standard NATO 5.56x45mm offering. Internally, a rotating bolt-action system was in play (as in the AR-15 / Colt M16's function), this coupled to a long-strike gas-operated system (akin to the famous Soviet / Russian Kalashnikov series of infantry assault weapons - namely the Ak-47). Some AR-15 parts (including the rotating bolt) were utilized in the IMP-221's makeup.

In the end, the IMP-221 was not an outright success for the USAF with trials of the prototypes having been had at the USAF Armament Laboratory (Elgin Air Force Base). The gun was not a vast improvement over current market solutions available to the service and there were revealed inherent issues in the overall arrangement and function of the weapon - particularly the sights coupled with the pivoting action and short length of the weapon. The bullpup nature also brought the fiery action and ejection closer to the face - a common issue of even modern bullpup designs.

Former SPECOPS player Mack Gwinn, Jr later acquired the rights to the weapon and reimagined it for the consumer market as the "Arm Pistol" under the Gwinn Firearms (of Bangor, Maine) brand label, this version now chambered in 5.56x45mm firing from a M16-style detachable box magazine. This form eventually ended in the hands of Bushmaster Firearms and the civilian market model went on to see limited sales between 1977 and 1988.

A version of the Bushmaster Armpistol is on full display in the extensive arms section of the USAF Armament Museum in Florida.©MilitaryFactory.com

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