I've got a copy of the US Army's "Concept Team" report on the Tunnel weapon, issued in 1970. This is the "44 Magnum" spoken of and seen above, a S&W M29 that was heavily modified to accept a machined-steel 15-pellet shot cartridge, and which was internally silenced by using a sliding piston inside the case (between the powder and the shot payload). The cartridges were loaded from the rear of the case, with the powder charge added after the payload and piston were in place, and then the primer (inside a separate piece) was screwed into place from the bottom. On firing, the piston was thrown forward, forcing the shot payload out of the cartridge and through the smooth barrel, but sealing the gas inside the case.
They had five of these revolvers made for testing in RVN, and almost 500 of the cartridges that went with them, and they were field-tested beginning in 1967, with the 1st, 25th, and Americal Divisions. They were apparently well-received, but all users reported wanting a lanyard added so they could hang them around their necks when they wanted both hands free. The biggest problem they found with these was the ammunition; 75% of users reported at least one misfire in service, but some of those rounds would go off on the second or third strike. Here are some of the photos included in the report:
Revolver and ammo:
Holster and ammo carriers: