According to Anthony G. Williams, the "API blowback principle is used virtually in all open bolt sub-machine guns" (technically, although generally these are known as "simple blowback" weapons due to the very late ignition of the cartridge compared to specially designed API blowback guns like the MK 108), although "the relatively low pressures and velocities mean that extended chambers and rebated-rim cartridges are not required" for sub-machine guns. In heavier weapons, advanced primer ignition (API) was originally developed by Reinhold Becker for use on the Becker Type M2 20 mm cannon. It became a feature of a wide range of designs that can be traced back to Becker's, including the Oerlikon cannon widely used as anti-aircraft weapons during World War II.