K bullet

Standard 7.92×57mm Mauser armor-piercing K bullet introduced in mid-1917. Note the tool-steel core protruding from the rear of the bullet to form a boat tail.
A tracer variation of the K bullet

The K bullet (from German 'Kern', core) was a 7.92×57mm Mauser armor-piercing bullet with a tool steel core designed to be fired from a standard Mauser rifle.