The present invention relates to a firearm shooting with an open breech and comprising a detent mechanism with a hammer block (5) that can block the forward movement of movable parts (2), a trigger (10) that can lift the hammer block (5) when the movable parts (2) move backwards, an action bar (12) that actuates the hammer block (5) and the trigger (10) when shooting is triggered and stopped, and a finger piece (13) linked to the action bar (12) by a disconnector (11) that can shift from a position in which the finger piece (13) is mechanically coupled to the action bar (12) to a position in which the finger piece (13) is mechanically uncoupled from the action bar (12), the action bar (12) shifting from a shooting position to a rest position when the disconnector (11) shifts from its coupling position to its uncoupling position so as to prevent the following shot from being fired if the finger piece (13) is maintained in the shooting position.