Q: Am I right in assuming that mid-range trajectory height is measured at half the distance from the muzzle of the rifle to the target? Also, is it true that rifles shoot always higher in hot weather. Bob Barker
A: The answer to your first question is no. The path (trajectory) of the bullet follows generally a parabolic curve more parabolic than circular. The highest point in this path above the muzzle- target line is actually approximately 55-percent of the range from the muzzle. Measured from line of sight, the high point moves even farther out. The higher the sights above the barrel the farther this point is from the muzzle. This shouldn’t have any effect in “holding under” for close shots since most shooters cannot estimate range nearly accurately enough to take advantage of the precise location of the high point on the bullet’s path.
Rifles do normally shoot higher in hot weather but to a much lesser degree than is commonly believed. Velocities are usually measured at 70 degrees in industry tests, or test results are corrected to the 70 degree value. The actual effect under normal conditions of temperature on velocity is known to be 1.7 fps per degree of temperature change with most stick-type powders. Consequently, if you are shooting a factory load listed at 3000fps in 100 degree F temperature it will develop 3051 fps (3000 + (1.7x30)).
If you’ve zeroed your rifle during 90 degree summer heat then take it out next winter at 20 degrees below zero, the bullet will travel 187 fps slower. This is enough to lower the point of impact measurably at longer ranges. Bear in mind , however, that it is the temperature of the ammunition, not of the air that produces this effect. Cartridges exposed to the sun’s rays may be 20-30 degrees hotter than the surrounding air, as will ammo in the gun exposed to the sun. ADI powders are not sensitive to changes in temperature which is a big advantage.