In the late 1960’s the Air Force Armament Laboratory began work on an improved aircrew survival weapon designed to be lethal yet small and lightweight. Colt Industries was given a contract to develop a weapon using the ’stockless rifle’ concept. The result was the GUU-4/P Individual Multi-Purpose weapon – IMP for short. Colt made five IMPs chambered to fire the .221 Remington Fireball cartridge.

This version was designed to provide a lighter round with more velocity. Remington developed this cartridge under contract. Cartridge weight was reduced by making the case body from plastic and joining it to a multi-piece steel head. Greater velocity was obtained by using a .17 caliber bullet carried in a .221 caliber plastic sabot. Six slots in the sabot cause it to separate from the bullet on muzzle exit. Remington made 2500 of these rounds and packed them in red, white and green 20 round cartons.

NOMENCLATURE : .221/.17 Plastic Steel
OVERALL LENGTH : 1.753″ TOTAL WEIGHT : 84.4 gr
BULLET                                                                                                           SABOT                                                              PRIMER
Type : full metal jacket                                                                                   Material : Clear plastic                                     Type : Small rifle
Diameter : .172″                                                                                             Diameter : .226″                                               Material : Nickel plated
Length : .528″                                                                                                 Length : .390″                                                    Sealing Lacquer : none
Weight : 24.8 gr                                                                                              Weight : 2.0 gr

CARTRIDGE CASE                                                                                       PROPELLANT
Headstamp : none                                                                                         Type : Rolled ball
Material : Blued steel (base) + mustard colored plastic (body)                Weight : 17.0 gr
Length : 1.395″
Base Length : .264″      
Rim Diameter : .373″     
Shoulder Diameter : .355″    
Neck Diameter : .251″
Weight : 40.7 gr (primed)