From the June 1955 American Rifleman, a question sent to the magazine from a reader regarding sighting a rifle when traveling to a higher altitude.
I realize that the sighting of a rifle must be changed when it is taken from low to high altitudes. However, if a rifle is sighted in at the base camp, how much should the sights be changed if the actual shooting is done 1,000 ft. above camp?—Filbert L. Mikesell, Grants Pass, Oregon.
Answer by E. H. Harrison: The correction depends on the bullet fired and its speed. With the caliber .30 M2 cartridge, which is a fair example, the difference in sight setting between sea level and 6,000 ft. elevation is about a minute and a half, when shooting at a range of 500 yds. It becomes proportionately much less at shorter ranges, being only about half a minute at 300 yds. It is seen that the effect is less than generally believed, and that an altitude change of 1,000 ft. would have no noticeable effect.
The above applies to the effects of changes in air density due to altitude alone.