This piece on five firearms that died at the prototype stage features the usual suspects — SPIW, HK G11, HK XM8, XM25 — and one I wasn’t familiar with, the Colt SCAMP:
SCAMP (which is such a rad name for a gun) stands for Small Caliber Machine Pistol.
The idea behind the SCAMP was similar to that of modern personal defense weapons: it was intended to arm officers, vehicle crews, and personnel who weren’t on the front lines. These personnel were often issued the M1911, a .45 caliber handgun; and the SCAMP gave the troops increased firepower over the M1911 thanks to its select fire.
Machine pistols are famously difficult to control, so to make the SCAMP easy to use, it was chambered in a new cartridge called the .22 SCAMP which was a fast 5.56x29mm round.
The small round resulted in light recoil; further, an integrated compensator helped increase the pistol’s overall control. The smaller round also allowed for a magazine capacity of 27 rounds.
By all accounts the SCAMP was a controllable and easy to shoot weapon. What sent it to the graveyard was that the military simply wasn’t willing to part with the M1911 at that time. Handguns are very rarely used in combat, so the investment into a new platform and ammo wasn’t an attractive prospect.