would only work if the gun was air-pressure powered. The legal definition of a gun is when you use gunpowder (or explosive) to propel the ammo. Air pressure technically is not explosive even at 4000 psi as far as the gun regulations are concerned. that being said, anything over 50 psi could cut right through you or rip parts off if it hits you just right, or grabs something else along the way. Take precautions if you dare to try this, and look up some of the currently sold air pressure guns to see the limitations they have.