ZSU-23-4 "Shilka" is a lightly armored, self-propelled, radar guided anti-aircraft weapon system (SPAAG). The acronym "ZSU" stands for Zenitnaya Samokhodnaya Ustanovka (Russian: Зенитная Самоходная Установка), meaning "anti-aircraft self-propelled mount"; the "23" signifies the bore diameter in millimeters; the "4" signifies the number of gun barrels. It is named after the Russian Shilka River. Afghan soldiers nicknamed it the "sewing machine" due to the sound of firing guns and because of the similarity of the name "Shilka" to the Russian word "Шилка". It is also referred to by its nickname of "Zeus."