Incline Shooting
Also: Angle Shooting · Rifleman's Rule
Uphill or downhill shots drop less than the slant range implies, corrected by multiplying that range by the cosine of the angle.
Incline shooting covers any shot fired steeply uphill or downhill, where the bullet drops less than the measured distance would suggest on flat ground. The reason is that gravity acts only over the horizontal portion of the flight path, so the effective range for bullet drop is shorter than the slant range read along the line of sight. Aiming as if the slant distance were flat sends the shot high, and the steeper the angle the larger the error.
The traditional fix is the Rifleman’s Rule, which multiplies the slant range by the cosine of the shooting angle to get a horizontal-equivalent range, then solves the trajectory for that shorter distance. Shooters read the angle with an angle cosine indicator or a ballistic app and apply the correction before dialing. The simple cosine method is a close approximation that loses some accuracy at extreme ranges and steep angles, where a full ballistic solver does better.