fills up the air storage chamber 13 and enters into the sleeve 21 of the spool 2 through the port 14 due to the opening 211 of the sleeve 21 blocked by the piston 22 the pressurized air continues gathering in the spool 2 as shown in fig3 upon a paintball b drops into the front end of the barrel 11 the user may pull the trigger 17 which drives the rod 185 of the three way valve 18 to move forward and then opens the blockage between the air inlet hole 181 and the air outlet hole 183 allowing the low pressure air to pass through the air inlet way 182 the air inlet hole 181 the air inlet hole 183 the air outlet way 184 the groove 31 of the low pressure regulator 3 and the key hole 15 into the through channel 25 of the spool 2 the air in the through channel 25 pushes the piston 22 to slide along the sleeve 21 forward and compresses the spring 221 when the piston 22 is moving forward the paintball b will be pushed to the triggering position and the opening 211 is cleared from blocking as shown in fig4 the high pressurized air accumulating in the spool 2 at this time will be discharged at a high speed from the opening 211 and spurred out of the through hole 24 of the piston 22 to fire the paintball b after the paintball b is shot the spring 221 urges the piston 22 to return to.