In a shell thrower, a grenade is inserted in a barrel and a booster charge under the grenade will fire and generate a gas pressure which forces the grenade out of the barrel. To improve the driving charge acting on the grenade, a pressure plate is detachably fixed under the grenade, and the booster charge is located on the underneath side of the pressure plate. An ignition mechanism for the booster charge is arranged on the underneath side of the pressure plate, which ignition mechanism is initiated by the impact of the pressure plate against the bottom or a shoulder of the barrel. A releasable engagement is arranged between the upper side of the pressure plate and the bottom of the grenade, which engagement includes a locking device, which is released from its locking position by a part of the driving pressure of the booster charge.