piston pressure?

[YobaRoba]

i have a copper piston gun: http://www.spudfiles.com/forums/viewtopic.php?t=410

i was wondering how much pressure should a piston of that size need to be moved back?, the piston has about 1 mm clearance around its self and the chamber.
i konw this is vague but its really hard to describe without making a mess of it.

[spudshot]

well i'd say go from like 5psi and work your way up, if it doesnt fire add some more pressure, when it first starts to fire thats how much pressure it needs to fire