wow you work fast! very good indeed! im impressed!
Im not sure but im guessing that a BM-21 would not be able to fire the rockets while moving, it would have to be at a complete stop and set up to fire. So the crew would have to get out of the cab and walk to the back of the vehicle or the side, to fire the rockets off.
Perhaps also when you have the targetting sorted you could make it so you can select how many rockets to fire like you did with the excellent 'Realistic KA-50', so you get a choice of different salvos of rockets in the options list.
The rockets are 122mm and have ranges of 11000m and 20300m.