Source: Ministry of Defense of Ukraine
A soldier belonging to the 243rd. Territorial Defense Battalion, 241st. Territorial Defense Brigade sits in the gunner's seat of a Zastava M55 20mm automatic cannon. The photograph was taken in the winter of 2023. The M55 is a Yugoslavian (now Serbian) license built Hispano-Suiza HS.804 20mm L/70 anti-aircraft cannon mounted on a Hispano-Suiza HS.630-3 towed carriage. Originally built by the Crvena Zastava arms company (now Zastava Arms), the single-barrel M55 (some sources designate it the 20/1 mm M55) first entered service with the Yugoslav People's Army in 1955. Despite the age of the M55, it remains in production as it is robust and dependable. Slovenia provided Ukraine with 200 M55 systems, both single-barrel and triple-barrel (known in some sources as the 20/3 mm M55), in 2022.
The M55 is chambered for the 20x110mm HS round and it has a gas operated, delayed blowback action. The M55, sans the carriage, has an empty weight of 94 pounds and with a full 60-round drum magazine, the weight increases to 151 pounds. The maximum rate of fire is between 600 to 700 rounds per minute with each round leaving the barrel at a muzzle velocity of 2,800 to 2,900 feet per second. With the muzzle brake, the recoil force is 881 pounds.
The 20x110mm HS cartridge case was brass with more modern production ammunition using steel cases. The standard HEI (High-Explosive Incendiary) round has approximately 32 grams of propellant with between 10.75 to 11.4 grams of explosive. The effective range against aerial targets is just under 1 mile (.9 of a mile) while the maximum range of the round is 2.7 miles. Fuzes include delayed-action and impact types. The M55 shown here is not fitted with any sights but the mount for an optical sight can be seen above the magazine.
The M55 can fire against ground targets if need be with an effective range of 1.2 miles. Today, the typical ammunition availability for the 20x110mm HS round (in addition to the HEI discussed above) includes HEI-T (High-Explosive Incendiary Tracer), HE-T (High-Explosive Tracer), and HE (High-Explosive). Armor-Piercing (AP) ammunition was made for the cannon but performance required the M55 to be at close range to be effective. The AP round could penetrate 15mm of hardened steel at a 40 degree slope at a range of 365 meters. A more effective APCR (Armor-Piercing Composite Rigid) round, using a tungsten core, was produced that allowed for around 38mm of armor penetration at 365 meters at a 30 degree slope.
As the M55 is manually operated, the cannon is mainly in service with anti-UAV units as the slow speed of drones such as the HESA Shahed 136 (around 115 miles per hour) are more easily tracked and targeted by such weapons.
