Diversity Is Good, Even In PBR

PBR (Point Blank Range) is a concept useful for hunters. It is the maximum range at which you “can’t miss” without adjusting sights for range. To the uninitiated, a bullet moves somewhat like a baseball, only faster. Despite great speed, a bullet does not travel in a straight line to a target but on a curved trajectory. The trajectory is not as simple as a parabola because the bullet slows down, losing energy to the air. The slower the bullet, the faster the bullet descends from the highest point on the trajectory. A hunter sends the bullet upwards a bit in order for it to reach the target at extended ranges. If you can hit the target at longer ranges, you are a more successful hunter, in theory.

The most common concept of PBR is that the hunter will take aim at the centre of the vital zone (hear/lungs/major vessels) and for part of the trajectory, the bullet will be above the line of sight and for part of the trajectory the bullet will descend to the bottom of the vital zone. The distance to the point where the bullet strikes at the bottom edge of the vital zone is considered the maximum PBR but it is not.

The reason is that at the middle ranges, the bullet is travelling parallel to Earth but later descends at an ever increasing rate. Using ranges after the bullet crosses the line of sight the second time (descending rather than ascending) greatly shortens the PBR. Look what happens if one sights at the bottom of the vital zone and calculates the range where the bullet descends back to the line of sight. For both curves, the bullet is staying within a 10 inch vital zone (say, 150 grain SP from a .308 Winchester and mature white-tailed deer).
See, the PBR with the 6 o’clock hold is ~40 yards further, about the distance it takes the bullet to drop 5 inches…

40 yards may not seem like much, but if you are out in some field, the area you can reach is $latex \frac{364^2}{324^2} X 100 = 26\%$ more. That could mean 3 days of hunting instead of 4 or 4 deer in the freezer instead of 3. It matters.

Where does diversity come in? There are many ways of sighting in a rifle. Hunters who think about the game, the terrain, and the typical distances they encounter game should choose the method that will work best. For example, if you have a really long ranging rifle and a steady rest, you can zero for very long ranges and hold under the vital zone to exploit the flat part of the trajectory out a very long way. With a 7mm Remington Magnum or anything like .30-’06, it is possible to use a heavy pointed bullet to deliver lethal energy to an elk or moose to 500 yards. The large size of these animals gives a larger vital zone. You could hold one “vital zone” down and do the job. Here’s a trajectory for a zero at 573 yards with 175 grain SPBT at 2900 ft/s. Now the hold is useful from 100 to 500 yards with no particular knowledge of the range except that it’s far and greater than 100 yards. That’s 0.31 square miles covered by a single hunter from a single position, barring hills, rocks and trees…

This is a lot like FLOSS (Free Libre Open Source Software). By doing things this way, a hunter gets the best performance out of his hardware.

About Robert Pogson

I am a retired teacher in Canada. I taught in the subject areas where I have worked for almost forty years: maths, physics, chemistry and computers. I love hunting, fishing, picking berries and mushrooms, too.
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