Of the type actuated by a trigger (9) having a support part (14), which defines a housing (1) which comprises at least one air inlet (2,12), at least one outlet (3), characterised in that it comprises:



a first body (5), as a valve, which, when in a rest position, blocks the air outlet (3);
a second body (6), as a piston, which partially encloses the first body (5), said second body (5) being blocked by the trigger (9) when in a rest position,
a first elastic means (8), arranged in the housing (1), between the second body (6) and the support part (14),
a slot (10), formed in the first body (5), and
a pin (11), crossing the slot (10), and which fits in certain holes (13) formed in the second body (6).