Take a look at the Technobots guide notes - http://www.technobots.co.uk/en-gb/pg_19.htmlhttp://www.technobots.co.uk/en-gb/pg_19.html theres two very good articles about LP and HP pneumatics.

However, it has to be said, people who dont know what theyre doing shouldnt play with compressed gas (dont think Im being condescending, I include myself in that statement!). Pnaumatics have the capacity to do a lot of damage to humans, so I, personally, avoid them at all costs. If youre not comfortable with pneumatics, theres plenty of weapons you can build without them.