Complex sentence: The 9x57mm Mauser is a cartridge based on the 7.92x57mm Mauser, and uses the identical 57 mm-long cartridge case, with the same shoulder angle, but necked up to accept a 9 mm-diameter bullet.
Split into simpler sentences: The 9x57mm Mauser is a cartridge based on the 7.92x57mm Mauser. It uses the identical 57 mm-long cartridge case, with the same shoulder angle, but necked up to accept a 9 mm-diameter bullet.