Here is the weapon that was used in the Vietnam War that most people are not aware of

The QSPR ‘Tunnel Weapon’. When US forces had to crawl into tunnels to chase the enemy it was a very intense experience. A GI often went down there with just a 1911 .45 and a flashlight, and once his buddies dropped him down there he was pretty much on his own. As you can imagine, firing a .45 in a space the size of a closet does your hearing no favors and also alerts other bad guys down there to your presence. The militay, in conjunction with AAC, came up with the ‘Tunnel Weapon’ a specially modified S&W .44 Magnum for the ‘tunnel rats’ to use.

It had a short barrel and a smooth bore like a shotgun. The gun used a special ‘captive piston’ ammunition that was almost perfectly silent. When fired, the propellant pushed a piston in the case forward, launching buckshot made from a special alloy. When the piston reached the end of the case, it was caught and did not leave the case. As a result, all the propellant gas was captured in the case … so no noise signature. The pellets, driven forward by the force of the piston, continued down the bore and out the muzzle. The result was a totally silent and flash-less weapon that could be used in a confined space like a tunnel. They never were distributed in any meaningful amount and only a few went into practical field trials. The guns almost never turn up but some of the special ammo shows up once in a while.