From the 1970s we have the 22 SCAMP (Small CAliber Machine Pistol) intended to replace the M1911 45 ACP as a close quarters personal defense weapon. From Colt, the gas-operated, autoloading pistol was chambered for a cartridge firing a 40 grain, 22 caliber bullet at 2100 fps. Prototype cartridges were made from re-formed 22 Hornet cases. Once the desired ballistics were obtained a contract was let for 1000 new cases and bullets to be loaded by Colt. (One source says only 500 of each were manufactured). Most of the cartridges were fired during testing making both prototypes very rare. It’s reported that only one prototype pistol was made. In the end, the concept was rejected by the military.
Below are shown the two prototype cartridges flanked by the 22 Hornet and 221 IMP, and the headstamps of each.