In general, 1/4 inch ID or larger air lines are needed for Guardair safety air pistols with 1/4 inch NPT inlets.
Air lines having a 3/8 inch ID or bigger are required for air weapons with 3/8 inch NPT inlets.
Air lines of a 3/4 inch ID or bigger are needed for air weapons having a 3/4 inch NPT input.
Air lines can always be designed larger than necessary, but not smaller, to assure rated thrust.