I found this belted 20mm case with a “Pinup girl” trenchart design on ebay earlier. It appears to be made at Haerens Arsenal, Copenhagen from the intertwined “HA” on the headstamp. What is it? The seller quotes the length as as “around 4 inches”. It would not let me post only the images as it says they are “protected”.
I do not believe that is the mark of H
John, you are right, this is a 20x105B Rheinmetall made in Hungary.
here’s a picture about this cartridge from an original Hungarian manual
This ammo was used in the Solothurn S18-100 series anti-tank rifles (the variants ran up to S18-500, I believe). These were used by Hungary (see pic, from this site: denes.us/forums/YaBB.pl?num=1165339249 )
One variant, the S18-350, was fitted into a flexible nose mounting in some Dutch Fokker T.V planes - still only semi-auto.
It’s worth noting that the larger Solothurn S18-1000/1100 used the “long Solothurn” 20x138B round.
The 20x105B ammo was also originally used in an experimental Rheinmetall aircraft cannon, the Lb 204. However, there were feeding problems so the gun was modified to take a new rimless 20x105 round, in which form it was known as the MG 204 and saw very limited service as a flexibly-mounted gun in some Luftwaffe seaplanes or flying boats.