A paintball gun provides a double-acting trigger apparatus including a pair of finger engagement surfaces against which a user of the gun may employ a pair of trigger fingers. The user of the paintball gun may effect a firing operation of the gun by using either one of the two fingers to pull the trigger. Consequently, rapid firing operations of the gun are effected by dithering the two trigger fingers in rapid alternating motions to apply pressure on and off the two trigger engagement surfaces. Further, the paintball gun includes a programmable, microprocessor-based controller allowing the gun to be fired in response to a chosen discreet event in time. This chosen discreet event may include the opening and/or closing of a pair of switch contacts, for example; or the making or breaking of an electrical circuit effected between conductive components of the paintball gun. Movements of the double-acting trigger apparatus will result in plural discreet events separated in time, so that plural rapid-sequence firing operations of the paintball gun may be accomplished. Accordingly, a very high cyclic rate of fire is possible with a semi-automatic paint ball gun according to this invention.