A weapon storage device (10) which is to be attached to an external face (8) of a hull (4) of a submarine (2) and which is capable of containing weapons (14), such as ammunition or weapon carriers, which are to be deployed from the storage device on receipt of a deployment signal transmitted from inside the submarine, includes a compensation element (16, 18, 30, 40) which allows the variation in mass of the storage device (10) resulting from the deployment of at least one weapon to be compensated for, partially or wholly, by replacing a volume of gas initially contained in the storage device by an identical volume of water introduced into the storage device.