this is complicated and interesting question.
first to know is the size of the magazine's gas tank...
V = Width X Lenght X Height
then, what kind of gas you use?
Green Gas (1000ml) is somewhere around 140-170 PSI
134a is around 80-100 PSI
and please note the figures is based on atmospheric pressure at sea level, and varies depending on Temperature.
and then you can connect the dots