An apparatus for firing a projectile, such as a paintball, that comprises a trigger frame assembly that is detachable from either a mechanical cartridge or an electronic cartridge. The trigger frame assembly includes a trigger, a sear, and a frame on which the trigger and the sear are mounted. The trigger has a mechanical actuating surface and an electronic actuating surface. Both the mechanical cartridge and the electronic cartridge are configured to selectively attach to and detach from the frame. It is an aspect of the invention to allow a user to selectively and readily convert the apparatus between mechanical firing actuation and electronic firing actuation.