Shooting Up Or Downhill
Shooting up a hill or down into a valley both will have the same impact, the bullet will hit high unless you adjust for angle. The bullet is shooting high because there are less gravitational forces acting on the bullet during its higher angled flight path. Gravity has a less of a downward force on the bullet while it is traveling at steep angle during its flight path.
For example, if you are shooting a 6.5 x 47 cartridge out to 600 yards along a flat shooting plane your elevation adjustment would be 13.4 MOA or 3.9 mils. If the angle of the hill or valley is at a 30 degree incline/decline than the elevation adjustment is 11.3 MOA or 3.3 Mils. If you failed to make this adjustment your bullet missed the target by 12 inches.
Most newer ballistic apps have a “Look Angle” adjustment where you angle your phone to the angle of the barrel and then recalculate your ballistic chart. But there is a simple math solution for angled shooting.
First determine the distance to the target, the cosine of the angle, and the bullet drop at the distance in inches shown on your ballistic chart.
A five-degree angle has a cosine of .99; a 10-degree angle .98; a 15-degree angle .96; a 20-degree angle .94; a 25-degree angle .91; and 30-degree angle has a cosine of .87.
The solution is (bullet drop at range x cosine = new bullet drop) then refer back to your ballistic chart and determine your new range by subtracting the new bullet drop.
For example, for a target at 500 yards with a bullet drop of 57 inches and you angle is 20 degrees; the math is: 57x.94=53.5 you need to adjust by 3.5 inches. This refined adjustment comes after subtracting the normal flat bullet drop, 57 inches from the cosine adjusted bullet drop, 53.5 in order to get the adjusted drop of 3.5 inches. The longer the distance and the steeper the angle, the more important it becomes to apply this formula.
A cosine indicator mounts to the scope usually as part of the anti-cant devices and most range finders also slope angle to the target.
A simpler calculation is the “quick fix” method that is used by a number of law enforcement agencies. The adjustment is based on an angle of 30 degrees and reducing the range by 10 percent. For example: 600 yards at 30 degrees you would shoot as if it was only 540 yards. By confirming this method with a ballistic calculator, a 10 percent adjustment keeps you on target with fair amount of precision.
The most important point is that shooting up or down the bullet will strike high unless you adjust your elevation reducing your distance by 10% keeps you on target.
President RTH Firearms