Rifleman's rule is a "rule of thumb" that allows a rifleman to accurately fire a rifle that has been calibrated for horizontal targets at uphill or downhill targets. The rule provides an equivalent horizontal range setting for engaging a target at a known uphill or downhill distance from the rifle (called the slant range). For a bullet to strike a target at a slant range of and an incline of, the rifle sight must be adjusted as if the shooter were aiming at a horizontal target at a range of . Figure 1 illustrates the shooting scenario. The rule holds for inclined and declined shooting (all angles measured with respect to horizontal).
Strictly speaking, the rifleman's rule is an approximation and it holds generally only for the small angles typically involved in shooting. The rule is derived assuming that the bullet travels in a vacuum. However, empirical evidence suggests that the rule does appear to work with reasonable accuracy in air and with both bullets and arrows.
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