High Quality Content by WIKIPEDIA articles! The ZSU-23-4 "Shilka" is a lightly armored, self-propelled, radar guided anti-aircraft weapon system (SPAAG). 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. Polish soldiers nicknamed it maszyna do szycia (sewing machine) due to the sound of firing guns and because the similarity of name "Shilka" and Russian word "шить" ("shic`") (to sew). It is also referred to by its nickname of "Zeus".