Shooting uphill and downhill

A deer stands 400 m away, measured by a rangefinder, at 30 degrees below you. Dial the 400 m correction and the bullet goes over its back. The rangefinder was not wrong and neither was the solver — the mistake was assuming the bullet cares about the 400 m.

Gravity only sees the horizontal

Gravity pulls straight down, perpendicular to the ground, and it deflects the bullet away from the line of the bore. When the bore is tilted, only part of the bullet's travel is across gravity's pull; the rest is along it, where it barely bends the path at all.

So the drop you must compensate for is set by the horizontal distance to the target, not the straight-line distance your rangefinder gives you. A 400 m shot at 30 degrees has a horizontal component of about 346 m, and 346 m is the number that determines your hold.

The rifleman's rule

The classic field method: multiply the measured slope distance by the cosine of the angle, then look up the drop for that shorter distance.

The practical threshold sits around 15 degrees. Below that the correction is smaller than the other errors in your solution; above it, ignoring the angle starts costing you the target.

Where the simple rule breaks down

The rifleman's rule is an approximation that assumes drag along the flight path is unchanged. That assumption holds well for short and moderate distances, and degrades as time of flight grows.

Two things it does not capture:

• Air density changes with elevation along the flight path. A steep uphill shot sends the bullet into progressively thinner air; a steep downhill shot into thicker. Over a long shot this shifts drag in a way the cosine rule cannot see.
• Sight-height geometry re-enters at steep angles. The offset between bore and scope no longer sits neatly perpendicular to the trajectory, which is one reason scope height matters more at angle than on the flat.

This is why a solver that computes the trajectory step by step in three dimensions, as this app does, will disagree slightly with a cosine multiplication at long range and steep angle — and the solver is closer to right.

Measuring the angle

Eye estimates of slope are notoriously poor; people routinely call a 15-degree slope 'about thirty'. Since the correction depends on the cosine, an overestimate makes you hold low and miss the other way.

1. Use an instrumentAn angle-capable rangefinder, a bubble clinometer, or the app's clinometer, which reads the phone's own tilt sensors.
2. Rest the phone or the device on the rifleReading the angle to the target means aligning with the bore, not with the hillside you are standing on.
3. Take the reading before you settle behind the rifleIt is one less thing to fumble once you are in position and the animal is moving.
4. Enter the angle, not a corrected distanceGive the solver the measured slope distance and the measured angle and let it do the geometry. Correcting the distance yourself and then also entering an angle applies the correction twice.

That last point catches people. Some rangefinders output an 'equivalent horizontal range' already corrected for angle. If you feed that number into a solver *and* tell it the angle, you have compensated twice and you will shoot low.

Wind at an angle

Wind does not follow the same rule. A crosswind acts on the bullet for the whole time of flight regardless of slope, so wind drift is governed by the actual distance travelled, not the horizontal component. Correcting wind by cosine is a mistake that gets less obvious the steeper the shot.

Frequently asked