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IMP-221 GUU-4/P

 The IMP-221 / GUU-4/P individual multi-purpose weapon. Note that the pistol grip is canted to the side to provide comfortable hold.
 The IMP-221 / GUU-4/P individual multi-purpose weapon. Note that the pistol grip is canted to the side to provide comfortable hold.

The semi-automatic Bushmaster
The semi-automatic Bushmaster "Armpistol", designed by Mack Gwinn in 1980s, followed the original concept of the IMP-221, but it was chambered for standard .223 Rem / 5.56×45 ammunition.




Caliber .221 Fireball (5.56×36)
Weight n/a
Length (stock closed/open) n/a
Barrel length n/a
Rate of fire 500 rounds per minute
Magazine capacity 10 or 30 rounds


The story of the IMP-221 Individual Multi-purpose Weapon (also known as GUU-4/P submachine gun) began in 1968, when US Air Force contracted Colt corporation for development of the air crew survival gun, designated as Individual Multi-purpose Weapon (US AF index GUU-4/P). The proposed weapon was required to have lethal range of not less than 100 meters, weight not more than 1,5 pounds (680 gram) and overall length not more than 13 inches (330 mm). The basic concept of the new gun was developed by the Dale Davis of US AF Armament Laboratory, and several prototypes were built by Colt. This gun never went into production, but it is important as the one of the earliest forerunners of the modern concept of small-caliber PDW / Personal Defense Weapons, such as FN P90, HK MP7 or MSMC.
The characteristics, required by US AF, were achieved by using relatively low-recoiling .221 Fireball (5.56×36) ammo, loaded with pointed bullets, and using the 'stockless' bull-pup layout. To achieve comfortable handling by both right- and left-hand shooters, the pistol grip was allowed to rotate (cant) sideways, and it had three fixed positions – vertically below the barrel, canted by 38 degrees to the left, or canted by same amount to the right. To provide sighting means despite the position of the pistol grip, the IMP-221 Individual Multi-purpose Weapon had three sets of iron sights, at about 10 o'clock, 12 o'clock and 2 o'clock positions. Gun was fired like a conventional pistol, in single shots of in full automatic mode.

The IMP-221 Individual Multi-purpose Weapon was a gas-operated, with gas piston running above the barrel. Locking was achieved by a rotating bolt. Gun fired from closed bolt, in semi- or full-automatic modes. To achieve a controllable rate of fire, it has an inertia-type rate reducer. Feed was from detachable box magazines, which are inserted from the bottom. Ejection of spent cases was trough the top of the action.