My uncle was an engineer at Aberdeen Proving Grounds on that project and described it to me briefly in the 1980's. The object was to have a hard-hitting firearm with no flash and little noise. It was intended not only for tunnel use, but as an assassination weapon.
He said it met those objectives, but told me nothing about its deployment.
Sources:
From Wikipedia, the free encyclopedia and from an article on pages 27 and 28 of a magazine entitled "Smith & Wesson Hanguns 2002" published for the 150th anniversary of Smith & Wesson, written by Garry James.
The Smith & Wesson Model 29 "Tunnel Gun" was also known as the Quiet Special Purpose Revolver (QSPR).
The "Tunnel Gun" is perhaps the most unusual military Smith & Wesson gun of all. It wasn't produced by S&W, but was a modified Model 29 .44 Magnum that had its cylinder bored out to handle a special multi-ball "quiet" cartridge.
Smith & Wesson Model 29 44 Magnum Tunnel Gun (QSPR). Because the QSPR was rare and unique, it was tightly controlled. No known articles have come into civilian hands. This image was a photo of a standard S&W Model 29 that was photoshopped by Maxim Popenker to show what the extremely rare QSPR gun would look like.
The mission of this pistol was to efficiently clear out the tunnels and burrows of the hiding Viet Cong. It was officially termed the QSPR (Quiet Special Purpose Revolver). The cartridge was the heart of the piece and was a true ordnance marvel. It consisted of a steel cartridge case, propellant, piston, 15 small lead balls held within a composition sabot, and a special primer.
A few S&W model 29s were rebuilt by the AAI corporation to make the QSPR. These had new, short smoothbore barrels 1.375 inch (35mm) in length, with 0.40 inch (10mm) bore, and with cylinder chambers reamed to accept QSPR ammunition which externally represented metal-cased .410 gauge shotgun shells, but internally worked as a piston to trap the gas. They were identical to the previous model 29s except for the gold-inlaid trademark on the side cover and a new internal lock mechanism.
The QSPR evolved from 1967 US Army requirements for a silenced, multi-projectile hand weapon for use by tunnel exploration personnel (also called "tunnel rats"), which operated against Vietnamese communist forces in the numerous tunnels dug by NVA and VC personnel.
The weapon concept was developed at US Army Land Warfare Laboratory at Aberdeen Proving Ground, in collaboration with AAI corporation, which was responsible for creation of the internally silenced ammunition, based on the "gas seal piston" concept. It was similar to the concept employed in a number of Soviet Spetsnaz weapons, firing PZ type internally silenced ammunition.
The whole concept of the internally silenced ammunition is rather old and starts in the pre-WW1 era, but practical results were achieved only during 1950s and 1960s, when chemical and metallurgical technologies finally permitted manufacture of actual ammunition.
Quiet Special Purpose Revolvers (QSPR) were based on commercially available Smith & Wesson Model 29 .44 Magnum revolvers, rebuilt by AAI to handle their special integrally silenced ammunition.
The QSPR ammunition was quite different from a .44 caliber cartridge. It had a machined steel case with a screw-in base. Primers were secured deeply in the cartridge base by a screw-in bushing and an additional anvil, which transferred the blow of the hammer to the primer. Cartridges produced for tests in 1971 had no intermediate anvils.
The small charge of gun powder was enclosed at the front and sides by the cup-shaped steel piston, which, upon discharge, was securely jammed at the mouth of the case by the internal thread. The QSPR ammunition fired fifteen tungsten balls (loaded into a plastic sabot), each weighting about 7.5 grain (0.5 gram), at muzzle velocity of about 730 fps (222 m/s), which resulted in total muzzle energy of about 135 ft-lbs (185 Joules).
Due to the nature of the round shot, the practical lethal range was estimated at about 30 feet (10 meters), which was sufficient for extremely cramped tunnels of Vietnam war.
The sound signature of QSPR round fired from a QSPR revolver was about 110 dB, or similar to that of traditionally silenced .22LR pistol. It must be noted that QSPR revolvers had no sights, as these were intended for use at point-blank ranges and in very low visibility conditions of tunnels.
The basic mechanism (double action trigger and swung-out cylinder) were retained from standard S&W revolvers, although there were some modifications done to the hammer, and new short smoothbore barrel was installed.
The first ten specimens of QSPR revolvers were delivered for field testing in Vietnam in mid-1969. Testing continued until late 1969, with several live fire encounters with NVA / VC personnel. It is interesting that most of these encounters were actually not in the tunnels but during the ambushes made by US special operation forces on NVA or VC trails. The field testing proved the extreme usefulness of the QSPR revolver but also identified a number of issues which required further improvement of both the gun and the ammunition.
QSPR improvement and testing program was initiated in 1970, and lasted through 1971. However, withdrawal of US forces in Vietnam caused the decline of interest in this and some other developments, and the QSPR program was quietly terminated in around 1972.
The total number of QSPR revolvers built is not known, but various sources estimate the number to be between 25 and 250 guns in total.
Compared to the contemporary Soviet equipment of the similar nature, such as the S4M silent pistol, the QSPR probably provided somewhat more firepower at point-blank ranges because of the higher muzzle velocity and bigger ammunition capacity, but it was also significantly heavier and bulkier. This is not surprising, as these guns filled different niches. The S4M was primarily a concealed-carry "spy gun", while QSPR was a holster-carry "short range ambush" weapon.
Smith & Wesson Model 29 Tunnel Gun Cartridge:
When fired, the propellant pushed the piston forward, propelling the balls and sabot from the barrel. As the piston remained in the case after firing, the smoke flash and report were virtually eliminated. When one remembers that the standard way to clear a tunnel was with a grenade, shotgun, or explosive charge, any one of which would result in a smokey, noisy impediment to entering the tunnel after the assault, the beauty of the thing can be appreciated.
