How do high pressure hand pumps used in PCP rifles work? --- David, Cebu, Philippines
Most "BB Guns" and "Pellet Guns" operate using either a spring, a CO2 cartridge or a hand pump mounted in the stock (where you pump the gun up for each shot). They shoot standard .177 caliber BBs or Pellets.
PCP stands for Pre-Charged Pneumatic. In this case the gun has a large 2,000 PSI or 3,000 PSI air reservoir on the gun and you pump it up using an external hand pump. The larger reservoir gives you many shots at very high pressure. BBs fire at 1,000 feet per second or more. You can see a typical PCP rifle and the pumping process here: How PCP Air Rifles Work
The pump works like a standard bicycle pump except for one difference. On a normal bicycle pump, the piston might be 1-inch in diameter. That means the face of the piston has a surface area of .78 square inches. At 100 PSI, the person pumping the pump must apply 78 pounds of pressure to the pump's handle. At 1,000 PSI, the person would need to apply 780 pounds of pressure, which obviously is not possible with a hand pump unless the person is quite obese. So the pump for a PCP rifle uses a very small piston, perhaps a quarter inch or less in diameter. Now the face of the piston is a fraction of a square inch, and a person applying "normal pressure" to the handle can create high-pressure air. The tradeoff is that you have to pump many times with a small piston. For more info see How Hydraulic Machines Work.