OF OERLIKONS AND OTHER THINGS……
© Anthony G Williams
This is about the 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 probably 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.
I will not go into the technicalities of how they worked, as this is described in detail in my book "Rapid Fire". This is instead 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 which saw German service, as both an aircraft and an AA gun, at the end of the First World War. SEMAG bought the design and produced a more powerful version firing a longer cartridge (the Type L). Oerlikon then took over and introduced the Type S, firing an even bigger cartridge. By the late 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 (110mm). 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 the Army as well) and remained in naval use into the 1980s. The USN also used this model. 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 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, 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.
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 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.
The Japanese Navy also experimented with a smaller-calibre gun between 1937 and 1942, amended to use a 14x101 RB cartridge, but this also did not see service.
A comparative photo of the various different 20mm API Blowback cartridges can be seen here.
A photo showing the difference between Minengeschoss and ordinary HE cartridges (from 'Rapid Fire') is here.
With thanks to Ted Bradstreet for providing additional information.
