The housing of a motion picture projector has a chamber for a portion of a film-containing cartridge. The inlet of the chamber is surrounded by walls at least one of which extends into the path of movement of a cartridge into the chamber to be displaced by and to thereupon bear aginst the inserted cartridge, either owing to its resiliency or under the action of a spring. The movable wall can be moved to a closing position in which it closes the inlet and it can be held in the closing position by a cover for the housing or by a locking bolt. The movable wall can constitute a centering plate for the inserted cartridge and can be mounted for movement in and counter to the direction of insertion of cartridges into the chamber.