All of the Grundig rounds that I have info on have a base diameter of 12.48mm (.491) and a rim diameter of 13.8mm(.543) case lengths of 45,52,60 and 60mm with neck diameters of 11.48mm (.451) Rim thickness was 1.20mm (.0472) The 7,62 x 54R rim at 1.60mm (.063) would need to made thinner and the case blown out to straight taper(?). A better choice of brass to use for cartridge forming might be the 45-90 basic brass and turning the rim to proper thickness. The proof mark of 3 gGBP over Stmg is for 3 grams (46.29 grains) of powder with a steel jacketed bullet. Given the above I would agree that what ever the rifle barrel is chambered for it is very likely to be one of the Grundig proprietary cartridges but a chamber cast is definitly in order on this one. Jeff
