3.4.3 Sighting in for a Change in Shooting Location
Both hunters and target shooters often face the problem of preparing for a hunt or target competition at a location much different in altitude and shooting conditions from their local shooting range where they sight in their guns. Again, an example will help with the explanation of this situation. Suppose a hunter with a 30-06 Springfield is preparing to hunt deer and elk in a western location where the altitude is around 7500 feet above sea level, but he or she lives and uses a target range at an altitude of about 500 feet. The shooter wants the rifle to be zeroed in at 250 yards at the hunting location. The question is, where should the gun be sighted in at the local target range to make this happen?
Infinity can be used to answer this question. The first step is to calculate a trajectory with a 250 yard zero range for the shooting conditions at the hunting location. Suppose the hunter handloads the Sierra .308″ dia 180 grain Spitzer Boat Tail GameKing bullet in the 30-06 to a muzzle velocity of 2500 fps. We enter this muzzle velocity, a 250 yard zero range, and other appropriate trajectory parameters in the “Trajectory Parameter” list inInfinity. Note that we can enter a 1.0 mph crosswind (from the 3:00 o’clock or 9:00 o’clock direction) just to obtain a crosswind sensitivity for the trajectory at the hunting location. In the “Environment Parameters” list, we will use the standard atmospheric conditions because the actual weather conditions in the hunting location cannot be predicted in advance, and we set the altitude at 7500 feet. Then, we calculate the trajectory.
On the top toolbar on the monitor screen, we then select “Trajectory Variations.” From the dropdown menu, we select “Environment Parameters” and make the following changes appropriate for the local target range. The barometric pressure is changed to 30.05 inches of Hg, the temperature to 75°F, the altitude to 500 feet, and the humidity to 65 percent, which are the local conditions for the day that the gun is sighted in. We then calculate the variations in the trajectory. Infinity tells us that if the gun centers groups 0.52 inch low at 250 yards at the local shooting range, the gun will be sighted in for a 250 zero range at the hunting location. Of course, this is a specific example, but this procedure can be used for any other set of shooting conditions.