Well as you can imagine the psi refers to pressure exerted with equal force over a square inch, and the copper units of pressure refers to the amount of pressure it takes to crush a calibrated copper cylinder of a known hardness, which then is measured and compared to a Tarage Table to convert the measurement to pressure.

"The 375HH is the greatest level of power you can get for the investment in recoil." (JJHack)
79s and losttrail, biggest waste of air.