you don't need to anything to the screw - that directs the gas hole either open for semi-auto or closed for single shot /grenade launcher- just use your combo tool wrench end on the gas cylinder- you may have to whang on it a couple of times to get to loosen or use a rubber mallet, but it will come off- i've even used a ratchet at times-it's not a metric socket- it's almost like they use a sealer of some type to secure the gas plug- once the gas plug is off,tilt the muzzle down and the piston will drop out the end- when you put the piston back in, make sure the tail goes through the hole on the back of the cylinder properly and has free travel up and down before you put the gas plug back on as it IS possible to assemble it wrong and not feed- i lock the bolt back when working on the gas cylinder so i can hear it "flop" back and forth- the rest of the strip is in fm23-8