You mean "Schedule 40" PVC, not "40 gauge" . A Google search for "Schedule 40 PVC" will give you suggested completions, including links to the schedule 40 spec. The pressure rating depends on the pipe diameter, so you need to read the spec (or read the printed information on the pipe itself).
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so 3 feet of 1/2 inch pvc
Non sequitur. You haven't specified what pressure you need to hold, so how can anyone agree or disagree with your claim of size? Did you read the specification for schedule 40? Did you see the column which tells you the maximum pressure each pipe size can hold? Have you made a note of RMS's caveat that PVC is not rated to withstand compressed air or gas, only liquids?
Also, PVC pressure rating decreases as the temperature rises, and PVC is not recommended for compressed air or gas, a gazillion Instructables for potato cannons notwithstanding.
thx again ill read the column