Abstract: A firearm mechanism, especially for automatic or semi-automatic shotguns, with a covering upwardly closed breech-block housing consisting of two mutually fitting and fixedly attached parts, an outer casing part made from sheet by pressing and a precision-cast inner part, which is stiffening the outer part and is provided with guiding surfaces for movable parts in the mechanism, e.g. a breech-block. The outer part is stiffened also by a lower mounting, closing the housing and having upstanding walls projecting in the housing. Cartridge feeding catches are arranged in the lower mounting, attached to its upstanding side walls.