This article is a summary of information in the book Rapid Fire: the Development of Automatic Cannon, Heavy Machine Guns and their Ammunition for Armies, Navies and Air Forces

Amended July 2013

Anthony G Williams 

This is a brief history of the Advanced Primer Ignition Blowback (or API Blowback) family of automatic cannon, which flourished between 1918 and 1945, played an important role in the Second World War, and some of which are still in service today. This type of gun is strongly associated with the Swiss Oerlikon company, but other firms and countries were involved in their development also.

The Advanced Primer Ignition Blowback

Before looking at the history, a brief explanation of the unique characteristics of API Blowback mechanisms. These are similar in construction to a simple blowback, in which the bolt or breechblock is held against the breech face only by its own inertia and a recoil spring. On firing a simple blowback, the high-pressure gas generated not only thrusts the projectile up the barrel, it also pushes the cartridge case, and therefore the bolt, backwards. The bolt is much heavier than the projectile and does not move very far before the projectile has left the muzzle, causing the gas pressure to drop sharply. So by the time a significant part of the cartridge case has moved backwards and is no longer supported by the chamber walls, the pressure is too low for it to burst. The momentum of the heavy bolt continues to carry it backwards, ejecting the fired case. The recoil spring then pushes it forwards again, chambering the next cartridge.

Simple blowbacks only work with low-powered cartridges (in practice, the smaller pistol rounds) because with powerful rifle or cannon ammunition the bolt would have to be so heavy for its inertia to hold it in place that the rate of fire would be extremely low - and if the gun were pointed upwards in the AA role, the weight of the bolt would overcome the recoil spring, causing to slide backwards. The API Blowback overcomes these problems in a mechanically simple way. The bolt is held back in its rearwards position before firing. When the trigger is pressed, the bolt is released and starts to move forwards, chambering a fresh cartridge. Just before it reaches its foremost position, the cartridge is fired. The gas pressure then has to overcome not just the inertia of the bolt but also its momentum as it travels forwards at high speed. This requires far more energy and means that the bolt can be a small fraction of the weight of a simple blowback.

A limited version of this is used in many sub-machine guns but to get the full benefit it is necessary to allow the cartridge to slide for some distance in an extended chamber, to make sure that it is supported by the chamber walls during the firing process and in the initial stages of rearward movement. This isn't possible with conventional ammunition since the bolt needs to have its extractor hooked around the rim, which if the rim is the same diameter as the case (as it conventionally is), means that the extractor will protrude beyond the diameter of the case so it won't fit in the chamber. To get around this, API Blowbacks have a cartridge case which has a rim of much smaller diameter than the case, so that the extractor can hook around it without protruding beyond the case diameter. This is known as a rebated rim and is essential to obtaining the full benefits of the APIB system. Rebated-rim cartridges are usually identified by the addition of RR or RB after the cartridge dimensions, as in 20x70RB (a 20mm diameter projectile with a case 70mm long and with a rebated rim).

The API Blowback History

This article is a commentary on the "API Blowback Family Tree" diagram which you can access here. As described in "Rapid Fire", the first API Blowback cannon was the 20mm Becker, designed by Reinhold Becker of Germany just before the First World War.  This saw German service in 20x70RB calibre, as both an aircraft and an AA gun, at the end of that war. SEMAG of Switzerland bought the design (Germany not being allowed to continue the development of such weapons) and developed a more powerful version firing a longer cartridge (the Type L). Oerlikon then took over in 1924 and introduced the Type S, firing an even bigger cartridge. By the end of the 1920s, the range consisted of guns built around three different 20mm cartridges; in order of increasing size and power, the Type F (70mm case length, later increased to 72mm), the Type L (99mm, later 100mm or 101mm in the Japanese Type 99-2) and the Type S (a wider case 110mm long). Their muzzle velocities were respectively around 600 m/s, 750 m/s and 820 m/s. These three ranges continued to be developed together.

In the mid-1930s, all three guns were offered in improved versions, suitable for aircraft use. They were known as the FFF (usually just FF), FFL and FFS respectively. Hispano Suiza produced modified versions of the FFS as the HS Type 7 and Type 9, modified for aircraft engine mounting. The Japanese acquired and experimented with the FFS as an aircraft gun but never used it in service. The British Royal Navy acquired the FFS as an AA gun, but in a heavier and sturdier version known as the Type SS. It saw extensive service throughout the Second World War (being used by British Army as well) and remained in RN use into the 1980s. It may still be found as a deck gun on warships of smaller navies. The USN also used this model extensively during WW2. Oerlikon continued to improve the SS range until after the Second World War, the last being the 2SS, capable of firing at 650 rpm. Polish engineers working in Britain produced a much simplified version of the SS known as the Polsten, which saw some service late in the War.

The FF was used in a range of aircraft during the mid-1930s, although only in small numbers. In 1937 the German Luftwaffe adopted a modified version of the FF, using an extended 20x80RB cartridge case, as the MG-FF, made under licence by Ikaria Werke of Berlin. This was the standard aircraft cannon for the first few years of the War and was improved into the MG-FFM (modified to fire high-capacity Minengeschoss HE ammunition) in 1940.

In 1939 the Japanese Navy also acquired the rights to the FF, although it was not used until 1941. Its most famous installation was probably in the Mitsubishi A6M Zero fighter, but it was used in a wide variety of aircraft. It was known as the 20mm Type 99-1.

The intermediate gun, the FFL, saw much less use. The only major user was the Japanese Navy, which again acquired the rights to produce it in 1939 but did not use it until 1942. This model in 20x101RB calibre, the Type 99-2, replaced the Type 99-1 in some applications where the higher muzzle velocity was required (e.g. later models of the Zero), but the Type 99-1 remained in service until the end of the War. In 1945 the Type 99-2 Model 5 was modified to achieve a significantly higher rate of fire - up to 750 rpm - but it did not reach service. There is more detailed information concerning the Oerlikon FFL in THIS article.

Most of the Oerlikon guns used drum feeds, typically of 60 rounds. However, 100 round drums saw service in the Model 3 versions of the Type 99-1 and 99-2, and the Type 99-1 and -2 Model 4 used belt feed. One installation of the MG-FF, for Luftwaffe night fighters, used a large-capacity power-driven belt feed. The Polsten used a 30-round double-row box feed.

Oerlikon also produced two shoulder-fired anti-tank rifles using the API blowback principle; the SSG and SSG-36, in 20x72RB and 20x110RB respectively. The layout of these weapons was different, as they had the mainspring behind the bolt instead of wrapped around the barrel, giving a particularly modern, uncluttered appearance.

Larger calibre weapons using the API Blowback operating principle were developed in Japan and Germany. The Japanese Navy's 30mm Type 2 was just a scaled-up Type 99-1. It saw limited use in fighter aircraft. The German 30mm MK 108 aircraft cannon, developed by Rheinmetall-Borsig, was much more important and armed many fighters at the end of the War. It was a more radical modification of the Oerlikon pattern and used a different layout. The MK 108 design was later scaled up to the 55mm MK 112 aircraft gun, but this did not see service. Perhaps the most peculiar Oerlikon derivative was the Japanese Army's Ho-301 aircraft gun which fired 40mm low-velocity caseless ammunition - with the propellant in the rear part of the projectile.

Ammunition for the Oerlikon family, from left to right:

20x70RB Becker; 20x72RB Oerlikon FF/IJN Type 99-1; 20x80RB Ikaria MG-FFM; 20x101RB Oerlikon FFL/IJN Type 99-2; 20x110RB Oerlikon Type S; 30x92RB IJN 30mm Type 2; 30x90RB Rheinmetall-Borsig MK 108; 55x175RB Rheinmetall-Borsig MK 112; 40mm IJA caseless


With thanks to Ted Bradstreet for providing additional information.