I have a set of 50 rounds that look like they are old about 30 (based on info below it looks more like 32) caliber rim-fire. The bullet diameter average is 7.8 mm or 0.307 inch. The measurements I made with an inexpensive caliber varied plus or minus 0.1 mm. So could be me or the instrument.
Length of the round was about 23.6 mm ( 0.929 inch) Case looks like copper is about 13.8 mm long by 7.6 mm diameter.
There is a headstamp of the letter D. It is not raised. Would this be Dupont?