I got this round with some other Oerlikons and Hispanos and noticed it’s a bit different. First thing I noticed was how heavy it was. It weighs 463 grams. It also has a very defined shoulder where the case necks and what appears to be thicker brass on the bottom inch or so of the case. I’ve never seen one like it and was hoping someone here could help.
Can you please provide all the information stamped on the driving of the projectile?
Not any direct information pertaining to the cartridge you show above but Paul Smith in his notebook series on experimental ammunition lists 2 experimental rounds from the 1950’s with joined brass cases. A 20mm necked down to .60 and a 20mm both having roughly a double length cartridge. In these two instances two cartridge cases were modified and joined end on to form a very long case.
MAT - 4 - 8 45 20MM M99
That’s all I can see on the driveband