HK CAWS

In 1982, the US military launched the Close Assault Weapons System program, which sought to develop a new combat shotgun with much greater range and firepower than the standard-issue Remington 870. The program stemmed from an earlier JSSAP program led by Carroll Childers called RHINO. The military decided to focus on improving soldiers' close-range firepower after experiences in the Vietnam War and the British military's involvement in the Malayan Emergency. Childers' goal was to create a select-fire 12 gauge shotgun with an effective range of up to 150 meters that was faster, more powerful, and easier to control than a conventional shotgun with military buckshot. Childers also stipulated a magazine capacity of 10 rounds, weight of less than 4 kg, armor-piercing projectiles, and a redesigned case that was incompatible with commercial 12 ga. shells, but the guns themselves had to be reverse-compatible with standard buckshot. Several companies submitted shotgun designs to CAWS throughout the 1980s. HK developed the bullpup shotgun seen here, Pancor developed the Jackhammer, AAI developed shotgun based on the M16, and Atchisson submitted the AA-12. Pancor dropped out of the trials due to financial troubles, and HK's entry would eventually win the trials against the other designs. However, the CAWS program was deemed unnecessary and closed at the end of the 1980s, leaving the HK CAWS stuck in the prototype stage.

Ian from Forgotten Weapons has yet to take a look at a CAWS, so I had to dig up all the following info on how it works from primary sources.

The HK CAWS is a gas-assisted, recoil-operated, select-fire bullpup combat shotgun with a recoil-reducing mechanism. It was designed by a team of German engineers led by Helmut Weldle, who also worked on the HK USP, HK P7, HK G36, and HK VP70. The ergonomics were based on the HK G11, which was also in development at the time. The gun was designed in November 1982 and patented on December 4, 1985 (I actually managed to dig up the patent here: https://patents.justia.com/patent/4635530) The patent goes into extreme detail about how the mechanism works, but I will summarize it here:

The action can best be described as a hybrid between long-recoil operation and short-recoil operation, or what I would call a "constant floating recoil" system. The barrel moves backward with the bolt and bolt carrier when the gun is fired, but they are not rigidly locked together for the entire length of travel. Unlike a conventional rotating bolt, the force to unlock it comes from the recoil spring itself, not gas pressure or inertia. The spring tension on the barrel is greater than that on the bolt carrier, and the barrel is slowed by a braking assembly, so initially both pieces move backward together and are locked by spring tension and momentum. However, the bolt is allowed to move backward slightly faster than