Rifle Terminology - Maximum PBR?
Q: You often mention the terms "optimum sighting-in distance" and "maximum pointblank range" in your articles. I have a rough idea of what these mean, but would appreciate it if you could explain them in more detail? Once I know how to go about achieving these goals I will know how to apply the science to my rifles with different loads.
A: The optimum sighting-in distance for a rifle depends on the cartridge, its muzzle velocity and trajectory, and on the ranges you expect to encounter in the field. If you'll be hunting in timber or where the longest shot your likely to take will be no more than 100 metres, the optimum sighting-in distance will be 100 metres or less, certainly no more than 150 metres. For extended range conditions such as open country in the mountains or on the plains you should sight-in your rifle to zero at 200 metres or more. But the optimum sight-in distance is directly related to the maximum pointblank range. (MPR) is that distance over which your bullet will not rise or fall more than a specified number of inches above or below the sighting point, so that an animals vital heart-lung area won't be missed. Put another way, the range at which you can shoot with a "dead-on hold" and still be assured of a clean kill. I included a table of point bank ranges for big-game cartridges in my Practical Reloading Manual that lists the bullet's height at 100yds., the zero range and maximum pointblank range. For instance, the 130gn .270 Win. load fired from a 24 inch barrel rises 3 in. above
the line of sight at 100yds., is zeroed at 278 and drops 4in. at 326. The highest point in its trajectory arc is 154 yd. Many other hunting loads have similar trjectories. there are computer ballistics programmes and mathematical formulas you can use to calculate the optimum sighting-in distances and maximum pointblank ranges of any given load. Or you can use the rule of thumb employed by many hunters of deer-sized game; simply sight-in your rifle to print 2-1/2 to 3 inches high at 100yd. If you are shooting a modern high-intensity hunting cartridge load or one of the magnums you can centre your crosswires behind the shoulder of a deer out to 275-300yds. and expect to drop game. If you have access to a range, it is wise to confirm the trajectory by shooting at targets both at short and longer ranges. This will tell you exactly where your bullets are hitting at each distance.