MK 1/QSPR/SPP-1.

This is coolbert:

NOT so much the weapon itself but the projectile as fired from the weapon!

1. "That’s Right the Navy had an Underwater Revolver"

"The M1 gun, also called the Mark 1 Mod 0 Underwater Defense Gun, is a firearm developed for underwater use by the United States military during the height of the Cold War with the Soviets. Like most other underwater guns, it was designed to fire a longer, streamlined projectile rather than a bullet."

Soviet naval special purpose units as you might correctly surmise also equipped with a similar underwater weapon.

2. SPP-1 underwater pistol (Russia).

"The SPP-1 (Spetsialnyj Podvodnyj Pistolet = Special Underwater Pistol) has been developed in late 1960s by the TSNIITOCHMASH organization on request from the Soviet Navy. The SPP-1 was intended for Soviet combat divers ('frogmen')."

Finally from that era of the Vietnam War an experimental firearm as to be used exclusively by American "tunnel rats". A firearm the flash, report and recoil mitigated, firing tungsten fléchettes with great lethality.

3. Quiet Special Purpose Revolver.

"Some Smith and Wesson Model 29s were rebuilt by the AAI Corporation to make the Quiet Special Purpose Revolvers (QSPR)."

"The cartridge is the real story behind this revolver. The captive piston round was capable of containing the blast noise, flash, and other energy normally done by the weapon. In essence, the round is the weapon and the revolver is simply the carrier."

ONLY about two dozen experimental models of the QSPR were made and to my knowledge never used in combat. Perhaps a devoted reader to the blog knows better?

coolbert.