As you would expect there is a long section on the development of revolver cannon, mainly postwar . This is the specific paragraph on the MG 213 development (found a couple of new pics of the MG 213C too):
"The million-point project was assigned the gun designation MG 213 (Maschinengewehr = machine gun). Politzer's successful design was given the third code letter assigned, and thus became the MG 213C. The cartridge had to be large to develop the required muzzle velocity and the case measured 20 x 135; it actually fired a 112 g shell at 1,050 m/s. While this was being developed, operational requirements dictated the development of a low-velocity 30 mm version in parallel; this became known as the MK 213/30 or just the MK 213 (Maschinenkanone = machine cannon); the 30 x 85 B round fired its 330 g shell at 530 m/s. Incidentally, it will be noted that Germany in the Second World War regarded 20 mm weapons as machine guns, not cannon, and surviving references to the 30 mm version of this gun sometimes describe it as the "MG 213/30"; as it was not adopted, the designation was never officially determined. The Luftwaffe wanted to use the MK 213 for bomber destroying, the MG 213C for bomber defence and for ground attack, the high-velocity 20 mm having a much longer accurate range as well as better armour penetration. Both guns (which weighed 75 kg; the 20 mm was 193 cm long, the 30 mm model 163 cm) could comfortably exceed the 1,000 rpm requirement, the 20 mm reaching 1,200 – 1,400 rpm, the 30 mm 1,100-1,200. Test aircraft installations were made, but the guns were a long way from being production-ready by the end of the war; just ten examples of the final design had been assembled."
Tony Williams: Military gun and ammunition
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