A firing module having a fully automatic shell supply mechanism for supplying shells to a heavy weapon in a housing. A shell ram having a loading tray is disposed on a free end of a shell transfer arm that is pivotable between a raised position, in which the loading tray is aligned with the gun bore axis, parallel to a plane of elevation, and a lowered position, in which the tray is essentially vertical. A gripping mechanism on a shell transport arm grasps a vertical shell stored in at least one ammunition magazine for supplying a shell from a shell transporter, to the loading tray, in the lowered position of the shell transfer arm. The ammunition magazines and transporter are disposed in the housing ahead of a trunnion. The loading tray is pivotable about a pivot axis, which is essentially vertical in the lowered position of the shell transfer arm, by at least 180.degree. between a receiving position opening ahead of the trunnion, and a delivery position opening behind the trunnion. At least one propellant charge magazine is disposed in the housing next to or behind the trunnion. A fully automatic propellant charge supply mechanism is disposed in the housing for supplying propellant charges to the weapon, and has a propellant charge supply tray pivotable into a region behind the weapon and in alignment with the gun bore axis.