More commonly known as 8mm Mauser. The ones in the picture are Turkish. You can see the “sickle & star” emblem on the headstamp in the third picture. Most of the surplus 8mm Mauser supply has dried up since very few if any nations still use the caliber for their military.
A Turkish sniper with an SVD. Aside from the gigantic optic, note the very bizarre bipod. It clamps onto the receiver like the original SVD bipod but the legs are farther ahead. Not sure if that is a Turkish design or not.