Abstract: A firearm has a barrel assembly and a breech block pivotally mounted with respect to the barrel assembly. The barrel assembly has a barrel block and a pair of barrels mounted in over and under relationship. The breech block has a generally forwardly projecting water table. Locking elements are provided for retaining the barrel assembly and breech block in desired relative relationship for firing with a portion of the locking elements being on the barrel assembly and a portion being on the breech block. In one embodiment rearwardly projecting key elements on the barrel assembly engage recesses on the breech block. A lever stem cooperates with shell head support members through cam means to urge the shell head support members forwardly when the firearm is in the firing position. Inertia firing pins may be provided in the shell head supports and intermediate firing pins may be provided in the lever stem.