I think neither tank air pressure, nor ambient air pressure is required to correct for ballistics.
Tank pressure is irrelevant if weight and muzzle speed are known.
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Boncuk.../QUOTE]
But, wouldn't tank pressure be the equivalent of powder charge? Isn't muzzle velocity "dependent" on the weight of the projectile and the tank pressure? These you can control before you fire, along with elevation and azimuth. Muzzle velocity is sort of an after-the-fact measurement, like the point of impact, good for future trajectory calculations if the weight and pressure stay the same.
Just musing 'til my morning coffee is brewed.
Ken