Does anyone know which type of cartridge was used to propel grenades in the Madsen 20 mm automatic cannon from the 1930’s? Also, which cartridge was used for the same purpose in later Hispano-Suiza 20 and 30 mm caliber guns?
I can only say something to the Madsen part of the question.
The Madsen/DISA books “Automatic Standard Arms of Modern Warfare” mention blanks with wooden bullets for this purpose. The books doesn’t make a difference between the 20 mm cannons and the machine guns.