Screwdrivers and the combo tool just doesnt seem to get the gas screw tight enough.
If you are having trouble with the Gas Screw getting loose:
First make sure threads are clean
If you dont have a Gas Screw tool get a 1/4 " drive extension and ratchet out of your tool box. It fits perfectly and allows you to get the screw tight. (of course wont work on single slot screw)

You should support the gas cylinder when tightening. If you dont have a gas cylinder wrench you can hold the cylinder with padded vicegrips ,adjustable wrench etc. This takes the torque off of the barrel splines