Immediately after the end of WW2, as the age of fast-moving jets dawned, most militaries around the world concluded that traditional anti-aircraft weaponry was rendered completely useless. Bombers flew higher and ground attack aircraft faster than traditional hand-powered AA guns could contend with, so most anti-aircraft weapons were withdrawn from service and destroyed for scrap while efforts were made to develop the first Surface-to-Air missiles to counter the threat of nuclear bombers.
By the end of the 50s however, the introduction of helicopters onto the battlefield forced militaries to re-think their abandonment of low-caliber battlefield anti-aircraft weaponry. The large, long-range surface-to-air missile systems they'd been developing were not suited for active battlefields to shoot down low altitude helicopters at close range, so some nations developed new weapons while others re-introduced some old favorites.
M45 (M55) Quadmount
The infamous "Maxson" from AS2 makes a return. The M45 Quadmount anti-aircraft turret (mounted on an M20 trailer to create the M55 Anti-Aircraft Weapon System) comprised 4 slaved M2 .50 caliber heavy machineguns. The continued use of the M2 machinegun meant that many M45s were kept in storage depots long-term, as they could be easily returned to service simply by reinstalling the weapons. It saw widespread service in Korea in the 1950s, and its longevity was such that many examples were taken out of storage and put back into action in Vietnam, primarily as a ground support weapon. In Europe it was one of the only anti-aircraft weapons ready to clear the skies of Soviet helicopters.
Hispano-Suiza HS.404
Seeing limited service in WW2, the 20mm Spanish-French HS.404 would be developed postwar in several applications, notably as the primary armament of the first generation West German Schutzenpanzers as well as a limited run of updated M114 reconnaisance vehicles. As such, it boasts moderate rate of fire, but was quickly outclassed by new post-war designs.
Rheinmetall Mk20
The Mk20 20mm was a new design that saw service beginning in the early 1970s with West Germany. Using the same 20x139mm ammunition as the HS.404, it had greatly improved rates of fire, barrel life and adaptability. Most often seen in a dual mount with the Bundeswehr, its most famous iteration was as the primary armament of the Marder IFV and Spahpanzer Luchs.
M167 Vulcan Air-Defense System (VADS)
The 20mm M167 VADS is a development of the M61 Vulcan rotary autocannon developed in the late 50s and which would become the primary projectile armament on US jet fighters. The decision to utilize a rotary firing mechanism allows the Vulcan to achieve mind-boggling rates of fire. For comparison, the Vulcan has about the same rate of fire as all four barrels of the Shilka's 23mm system combined. The towed VADS mounting entered service in the late 60s, but its more famous self-propelled version, the M163 based on an M113 chassis, entered service in the early 70s.
Oerlikon GDF
The Swiss-designed 35mm Oerlikon GDF, despite the fact that Switzerland was never part of NATO, would go on to see widespread use both in Europe and beyond. Particularly effective against the armoured attack helicopters that started entering service in the 1970s, the 60s-designed GDF is most often used in a dual mounting, either towed or, most famously, self-propelled as on the Flakpanzer Gepard and British 'Marksman' system.
Bofors L/70
The renowned Bofors 40mm anti-aircraft gun of WW2 would go on to serve into the 1950s in many, many armies, until replaced by the improved L/70. Boasting a higher shell velocity, doubled rate of fire, and the ability to integrate with radar-controlled systems, the L/70 would go on to be the primary medium AA gun of many NATO countries including Britain and West Germany, and its updated versions are still in service today.
ZPU-2 (ZU-2)
The ZPU-2 was developed immediately after WW2 by modifying the KPVT 14.5mm heavy MG into a light anti-aircraft weapon that could be easily carried by troops or mounted onto light vehicles when needed. Despite its larger caliber than the M45, the fact that it has only two barrels makes it roughly balanced with the American system. The ZU-2 as seen in-game was a lightweight modification for airborne troops.
ZPU-4
The ZPU-4 utilized the same 14.5mm KPVT heavy machinegun, but this time in a quad mounting. This would go on to be one of if not the most prolific anti-aircraft weapons in the world for the next 50 years.
ZU-23
In the 1960s, the Soviets decided to develop a new lightweight anti-aircraft weapon to replace the aging 14.5mm KPVT. They decided on 23x152mm for its caliber, 23mm being the most common autocannon size on Soviet jet aircraft (though the two are not interchangable in the slightest). Though it entered service well in the age of the helicopter, it was designed primarily as a high-velocity weapon for use against fast, low-flying jet attack aircraft. Like its counterpart the NATO 20mm, the ZU-23 often has both incendiary and armour-piercing rounds. Lastly, the ZU-23 would be used in a quad mounting for the ZSU-23-4 "Shilka" self-propelled anti-aircraft system.
AZP S-60
The 57mm S-60 was developed in the 1950s to replace all large-caliber automatic anti-aircraft guns; where the ZPU series would form the lower end of the anti-aircraft spectrum the AZP filled the middle, able to engage air targets at moderate ranges. Its relatively large caliber (and thus shell), and its high rate of fire, made it an effective weapon against ground targets. It saw service in the Soviet army until wholly replaced by missile systems in the mid to late 70s. Its most well-known development was as the primary weapon of the ZSU-57-2, in a dual mounting.
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