[vanir]

I've got an update on the revolver cannon used in the 410D modelling, which is fascinating stuff. I'd never actually heard of this gun and probably remembered it wrong as I went to the brilliant "military guns and ammunition forum" of Tony Williams. His website are easily the eminent source on the web of the topic.

Here was his reply posted this afternoon:

The MG 215 was made in two calibres: 13x92B and 15x83B. They are illustrated in the ammo photo gallery on my website. The intention was to replace the MG 131 with a simpler, cheaper and more powerful gun. It was a gas-operated Mauser design. It was dropped when interest focused on cannon calibres instead.

I think that the one you are referring to was the MG 213C, a revolver cannon developed initially in 20mm and then in 30mm. The 20mm fired a high-velocity 20x135 round (1,000 m/s), the 30mm a much lower-velocity (530 m/s) 30x85B. These were unique cartridges, not used by anything else. Both guns fired at c.1200 rpm. Only about ten prototype guns had been made by the end of the war.

After WW2, development of the MG 213C continued in several countries, leading (among others) to the Aden and DEFA 30mm guns which were initially close copies.