8mm Mauser Once-Fired Brass
From Wikipedia: https://en.wikipedia.org/wiki/7.92%C3%9757mm_Mauser
The 7.92×57mm Mauser (designated as the 8mm Mauser or 8×57mm by the SAAMI and 8 × 57 IS by the C.I.P.) is a rimless bottlenecked rifle cartridge. The 8mm Mauser cartridge was adopted by the German Empire in 1903–1905, and was the German service cartridge in both World Wars. In its day, the 8mm Mauser cartridge was one of the world's most popular military cartridges. In the 21st century it is still a popular sport and hunting cartridge that is factory-produced in Europe and the United States.