Question

The SHARP projectile weighs 10-20 kg. Why does the gun need recoil wagons, and why do you think the one behind the auxiliary barrel is 10 times heavier than the other one?

Answer 1

Conservation of momentum. When the gun fires, it's goal is to fire something into high altitudes from the ground, this kind of energy requires roughly escape velocity for the object upon leaving the Earth (ignoring friction). ~10km/s*15kg is ~150,000kg*m/s if the gun isn't *very* heavy, it's going to be knocked back very quickly (with v= 150,000/m).