A pneumatic fastener driver (10, 510, 1510) operable in a single sequential mode and a bump-fire mode, the pneumatic fastener driver comprising: a housing (18, 518, 1518); a nosepiece (26, 526, 1526) extending from the housing from which fasteners are ejected; a trigger (30, 530, 1530) moveable between a default position, in which a drive cycle is inhibited from initiating, and a depressed position, in which the drive cycle is permitted to be initiated; a contact arm (46, 546, 1546) movable relative to the nosepiece between an extended position and a retracted position; and a timeout mechanism (68, 568, 1568) operable in the bump-fire mode to inhibit (i) the drive cycle from being initiated, or (ii) actuation of the contact arm towards the retracted position, in response to inactivity of the contact arm over a preset time interval that begins once the trigger is actuated from the default position to the depressed position; a counting assembly (76, 576, 1576) that defines the preset time interval when the counting assembly is in an unexpired state, and wherein the counting assembly is in an expired state during inactivity of the counting assembly, and wherein the counting assembly is maintained in the unexpired state in response to actuation of the contact arm towards the retracted position before the preset time interval has elapsed; characterised in that: the timeout mechanism includes either: a) a mainspring (70, 570), a gear train that is driven by the mainspring during the unexpired state, and a lockout linkage moveable via the gear train to interfere with a portion of the trigger when the counting assembly is in the expired state; or b) a mainspring, a female barrel pivotably coupled around a pivot shaft of the trigger and driven by the mainspring during the unexpired state, and a lockout linkage moveable via the female barrel to interfere with a portion of the trigger when the counting assembly is in the expired state. a housing (18, 518, 1518); a nosepiece (26, 526, 1526) extending from the housing from which fasteners are ejected; a trigger (30, 530, 1530) moveable between a default position, in which a drive cycle is inhibited from initiating, and a depressed position, in which the drive cycle is permitted to be initiated; a contact arm (46, 546, 1546) movable relative to the nosepiece between an extended position and a retracted position; and a timeout mechanism (68, 568, 1568) operable in the bump-fire mode to inhibit (i) the drive cycle from being initiated, or (ii) actuation of the contact arm towards the retracted position, in response to inactivity of the contact arm over a preset time interval that begins once the trigger is actuated from the default position to the depressed position; (i) the drive cycle from being initiated, or (ii) actuation of the contact arm towards the retracted position, in response to inactivity of the contact arm over a preset time interval that begins once the trigger is actuated from the default position to the depressed position; a counting assembly (76, 576, 1576) that defines the preset time interval when the counting assembly is in an unexpired state, and wherein the counting assembly is in an expired state during inactivity of the counting assembly, and wherein the counting assembly is maintained in the unexpired state in response to actuation of the contact arm towards the retracted position before the preset time interval has elapsed; characterised in that: the timeout mechanism includes either: a) a mainspring (70, 570), a gear train that is driven by the mainspring during the unexpired state, and a lockout linkage moveable via the gear train to interfere with a portion of the trigger when the counting assembly is in the expired state; or b) a mainspring, a female barrel pivotably coupled around a pivot shaft of the trigger and driven by the mainspring during the unexpired state, and a lockout linkage moveable via the female barrel to interfere with a portion of the trigger when the counting assembly is in the expired state. a) a mainspring (70, 570), a gear train that is driven by the mainspring during the unexpired state, and a lockout linkage moveable via the gear train to interfere with a portion of the trigger when the counting assembly is in the expired state; or b) a mainspring, a female barrel pivotably coupled around a pivot shaft of the trigger and driven by the mainspring during the unexpired state, and a lockout linkage moveable via the female barrel to interfere with a portion of the trigger when the counting assembly is in the expired state.