Aircraft / Aviation Vehicles & Artillery Infantry Arms Warships & Submarines Military Pay Chart (2023) Military Ranks

Infantry Small Arms / The Warfighter

Bushmaster Arm Pistol

Bullpup Personal Defense Weapon (PDW) [ 1977 ]

Not quite pistol, not quite carbine - the limited-run Bushmaster Arm Pistol was born from an original USAF Vietnam War-era prototype design that went nowhere.

Authored By: Staff Writer | Last Edited: 01/26/2022 | Content ©www.MilitaryFactory.com | The following text is exclusive to this site.

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
Note: The above text is EXCLUSIVE to the site www.MilitaryFactory.com. It is the product of many hours of research and work made possible with the help of contributors, veterans, insiders, and topic specialists. If you happen upon this text anywhere else on the internet or in print, please let us know at MilitaryFactory AT gmail DOT com so that we may take appropriate action against the offender / offending site and continue to protect this original work.


Service Year

Untied States national flag graphic
Untied States


Bullpup Personal Defense Weapon (PDW)

Gwinn Firearms (Bangor, Maine) - USA / Bushmaster Firearms - USA
(View other Arms-Related Manufacturers)
National flag of the United States United States
(OPERATORS list includes past, present, and future operators when applicable)
Pistol / Sidearm
Compact design for close-quarters work or general self-defense.
Special Purpose
Special purpose weapon for a specially defined battlefield role.
Asymmetric Warfare
Design is highly variable / low-cost based on battlefield requirement and available materials; general use is in urban settings against an organized foe for high casualty count.

Compact nature allows for relatively easy concealment.
Despite its size / weight, the product can be transported by foot infantry.
Design sets the action behind the feed, allowing for a more compact design retaining a full-length, or near-full-length, barrel assembly.
Internal action allows release of multiple rounds with a trigger pull in quick succession.
An automatic, or pseudo-automatic, function is built into the system, allowing for repeat firing to be achieved.
Supports 'iron sights' allowing for inherent accuracy in ranged fire.
Design allows for user adjustability of certain components for customization.
Ease-of-use coupled with relative simplicity of combined components makes this entry a cost-effective solution for unconventional elements / fighting forces.

Overall Length
510 mm
20.08 in
Barrel Length
290 mm
11.42 in
Empty Wgt
4.30 lb
1.95 kg

Iron Front and Rear.


Gas-Operated, Rotating Bolt; Long-Stroke Gas Piston

Rotating Bolt
System utilizes internal mechanism to lock the breech or rear barrel assembly prior to firing.
Gas-operated system is featured, typically involving a gas cylinder and rear-driven piston directing energy to the bolt component.
(Material presented above is for historical and entertainment value and should not be construed as usable for hardware restoration, maintenance, or general operation - always consult official manufacturer sources for such information)


5.56x45mm NATO

Sample Visuals**

Graphical image of a 5.56mm intermediate rifle cartridge
Rounds / Feed

20-round detachable box magazine.
Cartridge relative size chart
*May not represent an exhuastive list; calibers are model-specific dependent, always consult official manufacturer sources.
**Graphics not to actual size; not all cartridges may be represented visually; graphics intended for general reference only.
Max Eff.Range
325 ft
(99 m | 108 yd)

Arm Pistol / Armpistol - Base Series Name.

Military lapel ribbon for the American Civil War
Military lapel ribbon for pioneering aircraft
Military lapel ribbon for the Cold War
Military lapel ribbon for the Korean War
Military lapel ribbon representing modern aircraft
Military lapel ribbon for the Ukranian-Russian War
Military lapel ribbon for the Vietnam War
Military lapel ribbon for the World War 1
Military lapel ribbon for the World War 2

Ribbon graphics not necessarily indicative of actual historical campaign ribbons. Ribbons are clickable to their respective campaigns / operations.

Images Gallery

1 / 1
Image copyright www.MilitaryFactory.com; No Reproduction Permitted.


Disclaimer | Privacy Policy | Cookies

2023 Military Pay Chart Military Ranks DoD Dictionary Conversion Calculators Military Alphabet Code Military Map Symbols

The "Military Factory" name and MilitaryFactory.com logo are registered ® U.S. trademarks protected by all applicable domestic and international intellectual property laws. All written content, illustrations, and photography are unique to this website (unless where indicated) and not for reuse/reproduction in any form. Material presented throughout this website is for historical and entertainment value only and should not to be construed as usable for hardware restoration, maintenance, or general operation. We do not sell any of the items showcased on this site. Please direct all other inquiries to militaryfactory AT gmail.com.

Part of a network of sites that includes GlobalFirepower, a data-driven property used in ranking the top military powers of the world, WDMMA.org (World Directory of Modern Military Aircraft), WDMMW.org (World Directory of Modern Military Warships), SR71blackbird.org, detailing the history of the world's most iconic spyplane, and MilitaryRibbons.info, cataloguing all American military medals and ribbons.

www.MilitaryFactory.com • All Rights Reserved • Content ©2003-