aren't all the modern gas rams sealed units that you can't alter the pressure on mate? If you've got one like a theoben ram with a valve in it I would say use a stirrup or scissor pump as it's easy to over fill them, the theoben ones don't take much pressure at all. There's a vast difference between the he and none he rams too in terms of operating pressure. Forgive me if I'm teaching my granny to suck eggs here by the way. Probably should have asked at the start, what ram you pressurising?