A modular weapon system of a preloaded recoilless gun including a barrel provided with a shell, a propellant charge and a countermass. The weapon system is configured in two parts including a first part of the barrel and a second part of the barrel. The first part of the barrel includes the shell and the second part of the barrel includes the propellant charge and countermass. A releasable locking device interconnects the second part of the barrel with the first part of the barrel.