First appearing in 1952, the ZSU-57-2 was a direct result of earlier developments based around the Soviet S-60 57mm anti-aircraft gun. Mounting a twin-barrelled version of the S-60, designated the S-68, it was a design that had been heavily influenced by WWII German research.
Upon entering service with the Soviet Armed Forces in 1957, the ZSU-57-2 was issued to the newly created air defence regiments of the tank and motorized rifle divisions.
Based on a lightened version of the T-54 tank chassis, it's components and armor were lighter than the tank version and so it was possible to modify the suspension and reduce the number of main road wheels to four. Hull armor was reduced to a level sufficient to defeat heavy machine-guns.
The S-68 gun mount was fitted into a very large and 'boxy' open-topped turret with little armor protection. Each gun had a maximum rate of fire of 240 rounds per minute and a practical rate of fire of 140 rounds per minute with 316 rounds being stowed in the vehicle (264 of these rounds in 'ready' clips).
The original versions were fitted with conventional optical sights without a range-finder, but the later models were fitted with a more sophisticated sight (identifiable by the two small 'ports' in the forward upper portion of the turret front). The ZSU-57-2 had an effective range of 4,000 meters (4,375 yards) and could be used to engage ground targets.
The ZSU-57-2 has a crew of six; a driver, two loaders, two gunners and a gun commander. A large screened basket for catching spent shell casings is fitted aft of the turret.
Since 1960 the ZSU-57-2 had been standard equipment throughout the Warsaw Pact countries and was exported to North Vietnam where it saw service in 1972 and again in 1973. Whilst the North Vietnamese used the vehicle for mobile convoy defence against roving American fighter-bombers, and occasionally used it against ground targets, it's primary role was the defence of PAVN armored units. Several batteries of ZSU-57-2's were attached to the 201st and 202nd Armored Regiments where, following Soviet practice, they served in air defence batteries in an armored regiment.
The main tactical limitation of the ZSU-57-2 was it's lack of onboard surveillance or fire-control radar. Range-finding depended on the use of of a mechanical computing sight with an optical reflex sight. Since it was dependent on visual target identification, this limited it's operations to clear days with good visibility. In Vietnam this was not too much of a problem as practically anything that was flying was likely to be American or VNAF and hence a target.
However, even supposing that the crew could identify a target at the outer effective tactical range of the gun system, a hostile aircraft travelling at 725km/h (450mph) was within the lethal envelope of the ZSU-57-2 for only 40 seconds (a long time for the pilot, no doubt) of which 5 seconds would be required for projectile flight time. This is a very short time in which to acquire, identify, track, range, engage, and fire on a target. A US Army study of the vulnerability of attack helicopters to the ZSU-57-2 concluded that it was only effective at relatively short ranges. The study indicated that the ZSU-57-2 firing a single burst had a probability of kill (Pk) of 48% at 1km, 14% at 1.8km and only 2% at 3km.