Russo-Ukrainian War: The Zastava M55 20mm Automatic Cannon

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.

Waifus with Raifus: The Rheinmetall-Borsig 30mm Maschinenkanone 108 Automatic Cannon

Artwork: Noriyasu Yamauchi

     The Rheinmetall-Borsig Maschinenkanone 108 (MK 108) 30mm automatic cannon first appeared in 1940, created by Rheinmetall-Borsig as a company endeavor. In 1942, the Reichsluftfahrtministerium (RLM; Reich Aviation Ministry) issued a requirement for a heavy cannon for aircraft which could engage Allied bombers and cripple or even destroy a bomber with minimal ammunition usage. Rheinmetall-Borsig dusted off the MK 108 and presented it to the RLM who, satisfied the cannon met the requirements, accepted the MK 108 for acquisition with the cannon entering operational service in mid-1943. The first aircraft to be equipped with it was the Messerschmitt Bf 110G-2 heavy fighter and the Bf 109G-6/U4.

     The MK 108, by itself, had a weight of 128 pounds with a barrel length of just under 2 feet (23 inches). The action utilized the principle of Advanced Primer Ignition (API) blowback. As is sometimes said of German engineering, this action required exceptional and precision construction and assembly to ensure smooth operation. API blowback works by igniting the primer within the round while the bolt is still moving forward and before the round is fully chambered. The rearward force of the detonating primer both slows the bolt's forward motion to prevent it from slamming into the breech while leaving enough momentum to send the bolt rearward to repeat the cycle. Such an action allowed the MK 108 to have a lighter bolt amd have a low recoil. 

     The MK 108 had a cyclic rate of fire of 650 rounds per minute with a muzzle velocity of 1,640 feet per second. This was rather low in comparison to other aircraft cannons and created a dangerously short effective range of 200 to 300 meters to ensure hits. While the MK 108 had a far longer maximum range, the round suffered from rapid drop which made accuracy at range poor. Testing showed that at 1,000 meters the round drop was as much as 135 feet from the aim point. However, the MK 108 was never designed to engage fighters (though, a single hit to a fighter could cause significant damage) but the much larger target profiles of bombers.

     The caliber of the MK 108 was 30x90RB. RB stands for Rebated which means the rim of the round has a smaller diameter than the base of the round. The main benefit of a rebated round is that it can be easily converted into differing ammunition without having to change the weapon's action mechanism. While the MK 108 had several ammunition types, the most common used in combat was the 30X90RB Minengeschoß (Mine-Shell) round. A thin-walled, steel cased round, it packed 85 grams of PETN (Pentaerythritol Tetraniotrate) explosive into the nose. Allied bomber crews were able to easily hear the MK 108's distinctive sound when it fired, nicknaming the weapon the “pneumatic hammer.”

     While the MK 108 had some success when equipping propeller driven aircraft, when the MK 108 was installed in turbojet aircraft, the short effective range made scoring hits a very dangerous affair for pilots. The Messerschmitt Me 262 jet fighters' speed was much higher than Allied bombers and so a successful engagement against bombers from the front or rear (an attack profile to minimize the number of defensive machine-guns that can take the aircraft under fire) was very difficult. With only 360 rounds to feed four MK 108 cannons, Me 262 pilots eventually developed an attack profile in which they flew above the bomber stream and once a target was selected, the pilot would put the Me 262 into a shallow dive which used the plane's speed to avoid escort fighters. Then, the pilot would continue the dive behind and below the bomber then pull up, bleeding off speed. This put the Me 262 around 1,000 meters from the rear of the bomber with the pilot opening fire at 500 meters then pulling away to avoid a collision at 200 meters. The rocket fighter, the Messerschmitt Me 163, had an even higher speed and pilots attacking bombers had between 2-3 seconds to execute an attack before having to pull away. 

     The lack of range saw the Me 262 equipped with 55mm R4M unguided rockets on underwing racks, 12 per wing. On approach, and outside of the bomber's heavy machine-gun range, the pilot would unleash the rockets which would create a spread pattern. Just one or two hits from a R4M was enough to down a bomber.

Russo-Ukrainian War: The Nudelman-Rikhter NR-30

Source: Reddit

     A Ukrainian gun truck crew, photographed sometime in 2025, looking after their weapon: the Nudelman-Rikhter NR-30 30mm autocannon. Entering service in 1954, the NR-30 is exclusively a weapon utilized in Soviet-era aircraft which included the Mikoyan-Gurevich MiG-19 (NATO reporting name Farmer), early models of the MiG-21 (Fishbed), the Sukhoi Su-7 (Fitter-A), and the Sukhoi Su-17 (Fitter). It is the latter where this weapon likely came from as the Ukrainian Air Force operated the Su-17M3 (Fitter-H), Su-17EM3 (Fitter-G), and Su-17M4 (Fitter-K) until 2004 when some 24 of the UM3 and M4 aircraft were overhauled and sold off to Yemen and Vietnam while the remainder were put into storage, dispersed between facilities in Odesa and Zaporizhia. As such, since these airframes are likely no longer airworthy or worth the effort to return to service, stripping their weapons for ground use is a logical option to “recycle” old components.

     The NR-30 is a short-recoil action autocannon with a weight of 145 pounds without ammunition. Speaking of ammunition, the NR-30 fires a 30x155mm round which was replaced by the 30X165mm round that remains in use to this day. As such, the NR-30 is not compatible with these rounds, making supply a potential issue since most of the Ukrainian Army uses the 30X165mm round in their BMP-2 IFVs, BTR-82A IFVs, BMP-3 IFVs, and 2K22 Tunguska SPAAGs. Likewise, the Ukrainian Air Force and Army Air Force use the round in the Mikoyan-Gurevich MiG-29 (Fulcrum), Sukhoi Su-25 (Frogfoot), and Mil Mi-24 helicopter gunships (Hind).

     Rate of fire is between 850 to 1,000 rounds per minute with the effective range (when mounted in aircraft) being a relatively short 200 to 800 meters against aerial targets and 1.1 miles against ground targets. Again, when used in aircraft, the NR-30 was usually provided with 70 rounds of belted ammunition. Typical ammunition types include the BR AP (Armor-Piercing), the OFZ HE-I (High Explosive Incendiary), and LP training projectiles. The OFZ round contains between 40 to 48 grams of explosive while all rounds had between 95 to 99 grams of VBP smokeless powder propellant.

     If one looks closely, one can see two blue gauges and a heavy hose coming from what is possibly a regulator valve. The hose runs underneath the custom fabricated gun mount. The valve is no doubt connected to a compressed air cylinder and the reason for this arrangement is because the NR-30 requires such an apparatus for operating the cocking cylinder to initially charge the weapon. Called pneumatic tensioning, the initial action triggers the recoil accelerator that unlocks the chamber and pushes the breech block backwards where it is held in place by a trigger. This action advances a round into the breech. The compressed air then releases the breech block forward, chambering the round, and the NR-30 is then ready to fire.