A method for rapidly firing a semi-automatic firing unit (22) having a trigger (24), a receiver (21) and a barrel (23). The firing unit (22) is placed in a handle (20) so as to enable only reciprocating linear movement along a constrained linear path (P). The user grasps the handle (20) and places their trigger finger (74) firmly on a finger rest (70). In use, the user generates a forward activation force (200) that urges the firing unit (22) forwardly so that the trigger (24) collides with the stabilized finger (74), stimulating the first round of ammunition in the receiver (21). A recoil force (202) from the discharging ammunition pushes the firing unit (22) rearwardly so that the trigger (24) separates from the stabilized finger (74). The intensity of the forward activation force (200) can be varied by the user on-the-fly to proportionally change the firing tempo.