In the 1980s, the United States Army enacted the Multi-Purpose Individual Munition (MPIM) program to better equip the standard infantryman with a more devastating urban assault weapon. One of the entries proposed during this period became the "Scorpion" which was produced by long-time aircraft-maker McDonnell Douglas and sponsored by the United States Army Missile Command service. The Scorpion was not adopted for several reasons.
The basic principle behind the Scorpion was a man-portable weapon system that could be fitted under the barrel of the standard-issue M16 assault rifle with the host weapon receiving sort of in-field modification. This resulted in a launch tube-like assembly, very much mimicking the form and function of the in-service, disposable M72 Light Anti-tank Weapon (LAW) (detailed elsewhere on this site). The Scorpion, also made disposable after a single use, was designed to be simple to operate and maintain and allowed the M16 rifle to retain its combat functionality to the fullest. Sighting was through the existing M16 sighting devices but the weapon held its own trigger unit and firing mechanism. Overall weight was 9lb when ready-to-fire and nearly 10lb when carried on-the-march. Overall length was 30.5 inches and the official caliber was 52mm.
In combat, the weapon could be used for breaching all sorts of materials - wood, brick, concrete (reinforced) - and featured a rate-of-fire of nearly five rounds-per-minute. It could also defeat earthen structures and the light armor of vehicles. Effective range was out to 985 feet with a muzzle velocity of 1,350 feet-per-second featured. Listed blast diameter was 20 feet.
However, several key deficiencies were apparent, chiefly its operating weight and length: at 9lb, it weighed more than the host rifle that was intended to carry it, effectively doubling the weight-carrying requirement of the infantryman. Its 30+ inch length also added to the 40-inch M16 rifle and about a quarter of the Scorpion assembly's length protruded ahead of the M16's muzzle - making wielding the complete weapon system in an urban setting unpractical.