Originally Posted by neftaly
I think you should be including height above sea level (atmospheric pressure) in the readings. Normalize them to 0m for comparisons.
95% of the population lives in altitudes below 4000 meters (Hypsographic demography: The distribution of human population by altitude
) and the difference between 0 and 4000 meters is only 40 kPa = 5.8 psi (Air Pressure and Altitude above Sea Level
So the differences caused by the altitude/atmospheric pressure are so small, that he plays no role. A little bigger effect can be perhaps on range, but not on muzzle velocity.