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From Wikipedia: “The Katyusha multiple rocket launcher is a type of rocket artillery first built and fielded by the Soviet Union in WWII. Multiple rocket launchers such as these deliver explosives to a target area more intensively than conventional artillery, but with lower accuracy and requiring a longer time to reload.
They are fragile compared to artillery guns, but are cheap, easy to produce, and usable on any chassis[ A Chevrolet 7107 truck, for instance]. The Katyushas of World War II, the first self-propelled artillery mass-produced by the Soviet Union,were usually mounted on ordinary trucks. This mobility gave the Katyusha, and other self-propelled artillery, another advantage: being able to deliver a large blow all at once, and then move before being located and attacked with counter-battery fire. Katyusha weapons of World War II included the BM-13 launcher, light BM-8, and heavy BM-31.