CAWS

The Heckler & Koch HK CAWS, known as CAW for short, is a prototype full-automatic, 10 round, 12 gauge, bullpup shotgun. It was briefly created as a joint effort by Heckler&Koch and Winchester/Olin.

Overview
During the Peace Walker Incident, Big Boss's mercenary unit, the Militaires Sans Frontieres, received design specs for the CAW weapons system as a reward for their services in aiding a group in capturing an enemy outpost, eliminating vehicle units, defending key bases, capturing enemy facilities, and capturing enemy bases during their missions outside Costa Rica.

Post-Peace Walker
During the 1980s, testing of the CAWS began to develop a new generation personal firearm capable of firing high-impulse, multiple projectiles within an effective range of 100-150 meters. Although it was tested by the United States Military, the project ended up cancelled and production halted.

In-game weapons description
"A fully automatic shotgun, created as a prototype to explore the potential of next-generation personal close-combat weapons. It fires a specially designed 12-gauge shell that allows for both high ammo capacity and a high rate of fire. This is the final word in shotguns, a gale-force barrage of lead in a compact package. If you're looking for maximum attack power at point-blank range, look no further - this is the weapon for you."

- CAW weapons description from Metal Gear Solid: Peace Walker