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 characteristics, required by US AF, were achieved by using relatively low-recoiling .221 Fireball (5.56x36) 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 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 / 5.56x45 ammunition.
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.
|Caliber||.221 Fireball (5.56x36)|
|Length (stock closed/open)||n/a|
|Rate of fire||500 rounds per minute|
|Magazine capacity||10 or 30 rounds|