A pneumatic assembly preferably includes a sealing member disposed between a compressed gas storage area and a compressed gas releasing area. A piston rod is preferably arranged in communication with the sealing member and disposed longitudinally in the pneumatic assembly extending between the compressed gas storage and compressed gas releasing areas. One or more vent channels are preferably formed in the piston rod, with one or more ribs arranged longitudinally through or between the vent channels. The ribs are preferably configured to maintain the sealing member within a sealing member retaining groove while compressed gas is being released from the compressed gas storage area to the compressed gas releasing area. A grip mounted circuit board arrangement can also be provided wherein a circuit board is configured to securely mount within one or more slots arranged in a grip frame of a paintball gun. The circuit board may include a solenoid valve and a trigger-actuated switch arranged on the circuit board. The paintball gun can further include an interchangeable external shell for housing the pneumatic assembly.