The third type of fast-firing cannon used in aircraft armament in the later 20th and early 21st centuries is the revolver cannon. Unlike the rotary-barrel cannon such as the M61 with its rotating assembly of barrels and the chain gun such as the M242 with its single barrel and a chain-driven breech assembly, the revolver cannon has a single barrel mated to a cylinder containing several chambers, like those of a revolver handgun, to speed the cycle of loading, firing and ejecting the spend case. Some revolver cannon are also externally power driven to further speed the loading, firing and ejecting cycle. Since the revolver cannon has just one barrel, the weight of the rotating element is less than in a rotary-barrel weapon, and this makes gas operation fully feasible.
There were many unsuccessful or impractical attempts to create a rapid-fire weapon from the early 18th century to the early 20th century before, in 1905, C. M. Clarke secured a US patent for the first fully automatic, gas-operated gun of the rotary-chamber type, but this design was ignored at the time as it appeared just as reciprocating-action automatic weapons such as the Maxim, Vickers and Browning water-cooled machine guns had been perfected and were becoming very popular in the world’s more advanced armies.
This it was not until a time in World War II that there emerged the first practical revolver cannon. The most important of these was a German weapon, the 20-mm Mauser MK213 and generally similar but scaled-up 30-mm MK213C which were on the verge of entering service as part of the armament of German fighters as the war ended. Almost all current revolver cannon are derived from Mauser’s ground-breaking design as, in the years immediately after the era, the Mauser engineers spread out from Germany and started to work for the armament designers and manufacturers of the countries which had won the war. The British and French made what were in effect straightforward copies of the 30-mm MK213C as the ADEN and DEFA types respectively, Switzerland produced the 30-mm Oerlikon KCA, and the USA manufactured the M29 as a 20-mm version rechambered for a slightly longer 102-mm (4.02-in) cartridge intermediate between the MK213’s 82-mm (3.23-in) cartridge and the 110-mm (4.33-in) cartridge of the Hispano-Suiza HS-404. So successful were the British ADEN and French DEFA weapons that the basic design was upgraded over several generations, but remained essentially unaltered into the 1970s.
New-generation revolver cannon
It was at about this time that a new generation of revolver cannon was developed on the basis of the proposed NATO 25-mm and Mauser 27-mm rounds. A leading example of this generation of revolver cannon is the Mauser BK27 used in aircraft such as the Panavia Tornado multi-role warplane. The BK27 is a gas-operated weapon with left- or right-hand linked belt or linkless feed to a five-chamber revolver cylinder, pyrotechnic recocking and electrical ignition. The ammunition options are AP, APHE, APHE-SD, HE, HE-SD, TP and TP-FP with a typical projectile weight of 260 g (9.17 oz) fired with a typical muzzle velocity of 1100 m (3,609 ft) per second at a selectable rate of 1,000 or 1,700 rounds per minute, though other rates are possible. The weapon weighs 100 kg (220.5 lb), and its dimensions include an overall length of 2.31 m (7 ft 6.9 in), barrel length of 1.70 m (5 ft 6.93 in) or 62.96 calibres, width of 0.296 m (11.65 in) and height of 0.246 m (9.69 in).
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.
With a single barrel mated to a cylinder with several chambers, the revolver cannon uses the revolver principle to accelerate the cycle of loading, firing and ejecting sequential rounds of ammunition, achieving a very high rate of fire compared to conventional single-barrel cannon of the same calibre but with a reciprocating action.
Pros and cons
Revolver cannon generally have a lower maximum sustained rate of fire than rotary-barrel cannon as all the rounds are fired through just one barrel, which therefore has to cope with considerably higher heating loads. Some rotary-barrel cannon have a rate of fire of up to 10,000 rounds per minute, but revolver cannon are typically limited to no more than 2,000 rounds per minute. However, on the other side of the tactical coin, the revolver cannon usually possesses a higher initial rate of fire as its moving mass is very considerably lighter than that of the rotary-barrel weapon, and as the rotary-barrel cannon’s external power source is less powerful in a burst than the gas operation of the revolver cannon.