Type Double Action revolver
Caliber(s) .40 smoothbore (.40 QSPR silenced ammunition)
Weight unloaded n/a
Length ~ 170 mm / 6.7"
Barrel length 35 mm / 1.375"
Cylinder capacity 6 rounds
Quiet Special Purpose Revolver (QSPR; also known as 'tunnel revolver' or 'tunnel gun') evolved from 1967 US Army requirements for a silenced, multi-projectile hand weapon for use by 'tunnel exploration personnel' (so 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 (similar concept at the time was 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. Earliest prototypes featured new, very short smoothbore barrels with .40" / 10mm bore, and with cylinder chambers reamed to accept QSPR ammunition which externally represented metal cased .410 gauge shotgun shells. Internally, however, the QSPR ammunition was quite different; it had a machined steel case with screw-in base. Primers were secured deeply in the cartridge base by screw-in bushing and 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 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 (tungsten 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 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 cylinedr) were retained from standard S&W revolvers, although there were some modifications done to the hammer, and new short smoothbore barrel was installed.
First ten specimen 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 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. Total number of QSPR revolvers built is not known, and various sources estimate that number between 25 and 250 guns in total.
Compared to the contemporary Soviet equipment of the similar nature, such as S4M silent pistol, the QSPR most 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 being primarily a concealed-carry "spy gun", while QSPR was a holster-carry "short range ambush" weapon.