3.4.2 Determining Zero Range from Firing Test Results
This is the opposite question to the one immediately above, and it also occurs frequently. In this situation, a shooter knows that the center point of groups fired from his gun at a local target range are located a certain amount above the point of aim at a measured range distance. The question then is what is the zero range of the gun?
Infinity can answer this question. We will use a specific example to illustrate how this question is answered. Suppose a shooter has a rifle chambered for the 25-06 Remington cartridge and uses Federal factory 25-06 Remington ammunition with the 117 grain Spitzer Boat Tail GameKing bullet. The shooter’s local target range is 610 ft above sea level, and he or she shoots on a pleasant day when the local atmospheric conditions are 30.05 inches of Hg barometric pressure, 75°F, and 60 percent relative humidity. The center of the groups fired is 3.25 inches high at 100 yards.
In the “Normal Trajectory” Operations list ofInfinity, we first go to the “Load Bullet” library and select Federal Cartridges. From the list of Federal cartridges, we select the 257 caliber and then select the 25-06 cartridge with the 117 grain Spitzer Boat Tail GameKing bullet, placing that bullet in the active bullet list on the monitor. Then, we prepare to calculate a reference trajectory for this cartridge under the conditions at the shooter’s local target range. In the “Trajectory Parameters” list, the muzzle velocity is 2990 fps (specified by Federal), the maximum range is set at 600 yards, the range increment for the print list is set at 50 yards, the zero range (for this reference trajectory calculation) is set at 200 yards, the elevation angle is set at 0 degrees (level fire), and the sight height is set to 1.5 inches for a telescope sight with a 40 mm objective lens. After these numbers are entered, we accept all the trajectory parameter values listed in the sidebar on the monitor.
In the “Environment Parameters” list, the barometric pressure is set to 30.05 inches of Hg, the temperature is entered as 75°F, the altitude is entered as 610 feet, and the relative humidity is entered as 60 percent. The wind velocity is entered as 0 mph, since there is little or no wind on this day at the target range. All the values of the environment parameters in the sidebar on the monitor are then accepted, and a reference trajectory is then calculated.
In the Operations list of Infinity, we select the “Calculate Zero” Operation, and enter the measured range of 100 yards and 3.25 inches above the line of sight. After accepting these values and pushing the “Calculate” button, the monitor shows that the zero range is 274 yards for this particular situation. Of course, this is just an example. The same procedure can be repeated for any set of shooting conditions or firing test results.