After the success of the BM-13-16 "Katyusha" in 1941, wich are 16 rails with 130mm M-13 rockets built on a truck (in FHSw on a Studebaker US6 U3 truck), the need for a larger rocket arose one year later. It saw his debut in the war in 1944 and also on Forgotten Hope Secret Weapon since version 0.4.
This was the BM-31-12 and was, like the name say, 12 rails with each an 300mm M-31 rockets. The rails are actually no steel rails but just a wooden frame where the rocket is place inside the frame. Based on the German Wurfrahmen, they have the nickname Rama. The design starts with the M-30. But a request for a rocket with a longer distance lead to the M-31 wich was based in the M-30. But still, the design was the same. The rockets are fired from the Rama. Only the distance are upgraded from 2,800 m to 4,325 m with the same 28.9 kg warhead. In March 1944, twelve Rama's are be built on a Studebaker US6 chassis wich becomes the BM-31-12 "Andryusha". This combination proved extremely successful in the further course of war, in particular during the fierce street fightings of Budapest and Berlin.
(* = the name BM-x-y are used for ground vehicles with the Katyusha rockets. 31 is for the rocket the (M-31) wich nearly show us the diameter. But with this rocket, it was 1cm smaller than the name. 12 are the count of rails. In particular, BM-31-12 is a vehicle which fires M-31 missiles and has 12 rails).