The ambient pressure indeed reduces the peak pressure and the velocity of a Springer, and in fact a Pumper as well (at any given number of pumps).... It does not affect a PCP filled to any given GAUGE pressure, (or CO2, which is temperature dependant) because that is compensated for by the gauge (it is psi above ambient).... Once fired, regardless of the gun firing it, the pellet will meet with less air resistance at higher altitude, and carry a larger percentage of it's velocity and energy to the target.... but starting from a lower velocity in the case of a Springer or Pumper....

Bob