A revolver cannon is a type of autocannon, commonly used as an aircraft gun. It uses a cylinder with multiple chambers, like those of a revolver handgun, to speed up the loading-firing-ejection cycle. Some examples are also power-driven, to further speed the loading process. Unlike a rotary cannon, a revolver cannon has only a single barrel, thus its spun weight is lower. Automatic revolver cannons have been produced by many different manufacturers.
The Confederate States of America used a single 2-inch, 5-shot revolver cannon with manually rotated chambers during the Siege of Petersburg. The gun was captured in Danville, VA by Union forces on April 27, 1865.
In 1905, C. M. Clarke patented the first fully automatic, gas-operated rotary chamber gun, but his design was ignored at the time. Clarke's patent came as reciprocating-action automatic weapons like the Maxim gun and the Browning gun were peaking in popularity.
In 1932, the Soviet ShKAS machine gun, 7.62 mm calibre aircraft ordnance used a twelve-round capacity, revolver-style feed mechanism with a single barrel and single chamber, to achieve firing rates of well over 1800 rounds per minute, and as high as 3,000 rounds per minute in special test versions in 1939, all operating from internal gas-operated reloading. Some 150,000 ShKAS weapons were produced for arming Soviet military aircraft through 1945.
It was not until the mid-1940s that the first practical revolver cannon emerged.
The archetypal revolver cannon is the Mauser MK 213, from which almost all current weapons are derived. In the immediate post-war era, Mauser engineers spread out from Germany and developed similar weapons around the world. Both the British and French made outright copies of the 30 mm versions of the MK 213, as the ADEN and DEFA, respectively. Switzerland produced the Oerlikon KCA. The American M39 cannon used the 20 mm version, re-chambered for a slightly longer 102 mm cartridge, intermediate between the 213's 82 mm and Hispano-Suiza HS.404's 110 mm. Several generations of the basic ADEN/DEFA weapons followed, remaining largely unchanged into the 1970s.
Around that time, a new generation of weapons developed, based on the proposed NATO 25 mm caliber standard and the Mauser 27 mm round. A leading example is the Mauser BK-27. In the 1980s, the French developed the GIAT 30, a newer generation power-driven revolver cannon. The Rheinmetall RMK30 modifies the GIAT system further, by venting the gas to the rear to eliminate recoil.
Larger experimental weapons have also been developed for anti-aircraft use, like the Anglo-Swiss twin barrel but single chamber 42 mm Oerlikon RK 421 given the code name "Red King" and the related single-barrel "Red Queen" - all of which were cancelled during development. The largest to see service is the Rheinmetall Millennium 35 mm Naval Gun System.
Soviet revolver cannon are less common than Western ones, especially on aircraft. A mechanism for a Soviet revolver-based machine gun was patented in 1944. The virtually unknown Rikhter R-23 was fitted only to some Tu-22 models, but later abandoned in favor of the two-barrel, Gast gun Gryazev-Shipunov GSh-23 in the Tu-22M. The Rikhter R-23 does have the distinction of being fired from the space station Salyut 3. The Soviet navy has also adopted a revolver design, the NN-30, typically in a dual mount in the AK-230 turret.
With a single barrel mated to a cylinder with multiple chambers, this type of autocannon uses the revolver principle to accelerate the cycle of loading, firing and ejecting multiple rounds of ammunition, achieving a very high rate of fire compared to conventional cannon of the same calibre.
Automatic revolver cannons generally have a lower maximum sustained rate of fire than rotary or 'Vulcan' cannons because their barrel suffers from much higher heating loads, as it alone must fire every round. Some rotary autocannons have a rate of fire of up to 10,000 rounds per minute (such as the Gryazev-Shipunov GSh-6-23), while revolver cannons typically have a rate of fire no more than 2,000 rounds per minute. However, revolver cannons are generally able to be made much lighter than rotary autocannons, requiring less support and mounting hardware -- rotary autocannons spin the whole multiple barrel and breech assembly, which, in equal caliber versions, can weigh hundreds of kilograms more in comparison (though the weight per rounds fired is lower for the rotary). The firing rate of a rotary autocannon is directly related to the rotational speed of the barrel cluster. The need to accelerate this cluster -- generally requiring a large, external power supply -- means that the maximum attainable rate of fire is not immediately available. In addition, rotaries suffer from lower accuracy, a consequence of dispersion caused by multiple barrels rotating at a varying speed. As a result of their design, revolver cannons lack these issues altogether.