O'Connor's Practical Field Ballistics

January 31, 2011 By Allan Jones

I've always considered myself a practical person, and I've felt that trait extends to my shooting and reloading. I've encountered too many shooters who get so tangled in the minutia that they end up with information overload.

By Allan Jones

There were landmark moments leading to this practicality. Realizing that a bullet's path is not an arc (a segment of a circle with a constant center), but rather a segment of a parabola, a curved line whose "apparent" center changes, was a big first step. The second was understanding that this parabolic path gets steeper the farther the bullet travels; the path is always flatter between the muzzle and the sight-in distance than it can ever be beyond that point.

Only recently did I fully understand how these ideas formed, as they are by no means my creative works. The seed was planted by the words of Jack O'Connor that I probably read in my preteen years. Jack O'Connor (1902-1978) probably wrote more on shooting and hunting than any other outdoor writer, and he had a huge influence on how we shoot and hunt today.

It was when I found some of my ancient O'Connor hardbacks while looking for another book that I began reacquainting myself with this very wise and practical man. In words I'd first read 50 years ago, I rediscovered a practicality that surely shaped the way I think about shooting.

As I reread these books, there were several points that I realized I had embraced. The first was basically, "Don't sight 'short' if your rife can do more." O'Connor talked about a .270 Winchester shooter he encountered who only sighted for 100 yards because he did not expect to get a longer shot than that. This goes back to the principle that the path between muzzle and sight-in point is flatter than the path beyond the sight-in point. With a longer sight-in, the guy could have been "on" at much greater distances without correcting elevation.

Another concept O'Connor embraced was that every animal has an area--not a point--where a bullet has a high chance of stopping it. The bigger the animal, the bigger that area. A whitetail may have an 8-inch vital area, but the same area on a moose could be 18 to 24 inches. When writing of the long-range rifle, O'Connor states, "For mountain use the rifle should always be sighted-in for the longest possible range that will not give midrange misses." (The Rifle Book: Second Edition, Revised; Knopf; 1972; page 274).

A "midrange miss" means the maximum rise of the bullet path above line of sight is greater than the area in which the bullet must land at some intermediate distance. If you are sighted for 400 yards and the bullet is a foot high at, say 225 yards, you could totally miss the small vital zone of a deer or antelope at the shorter distance.

This illustrates the mechanics of the maximum point-blank range (MPBR) concept.

Maximum Point-Blank Range (MPBR)

Primed in my youth by O'Connor's practical approach, I jumped all over this MPBR concept when it reemerged in the late 1990s. I beta-tested an exterior ballistics program that, in addition to figuring the usually bullet path values from ballistic coefficient and velocity, had a field to enter "Target Diameter."

This was the same as "vital area on the critter." If you entered a value of 8 inches for example, a graph showed dotted lines at +4 and -4 inches, denoting the vital area in side (path) view. It was easy to spot where the bullet went above or below the vital zone. There was even a little button that, when clicked, automatically adjusted the sight-in values so the bullet's path stayed just within the target diameter value; it told you the longest distance you could shoot without the bullet flying outside the zone. A window at the top of the screen showed how high you needed to be at 100 yards to keep the bullet in the target zone--handy if that's the longest shooting range you have in the neighborhood.

This was where modern technology and O'Connor's practical approach of the precomputer era came together for me. Jack figured out these ranging issues by pulling triggers and putting a lot of bullets downrange. He knew his rifles, the game, and the terrain, and he reached the same decisions we can today with the aid of computers. He even figured out how switching from a shooting bench to his commonly used field position--sitting--affected the trajectory. That's a remarkable achievement.

Maximum point-blank range is, in the concept of targeting a zone or area on an animal, simply the greatest distance that allows the bullet to neither rise nor fall more than one-half the target diameter relative to line of sight. Imagine a long tube whose inside diameter is the same as the vital zone. Your rifle is at one end of the tube, and the other end touches your quarry. When you shoot through this tube, the bullet must not hit the top of the tube at midrange and must clear the far end of the tube by a hair's breadth (see the illustration). If you shoot to a zone or area, MPBR is the length of the tube, or how far you can shoot without elevation correction. That's one less thing to worry about.

I think my fondness of this concept comes not so much from its benefit for long-range shooting but at the other end. I've always had more trouble with closer shots where hold-under is the key. Sighting for MPBR largely takes away this concern.

That computer program I beta-tested evolved into RCBS.LOAD, and it retains that useful MPBR function. However, you can use almost any trajectory program that allows you to set up tables in 25-yard increments. This is sufficient resolution to show you where the bullet achieves a rise and fall of one-half target diameter. It will show where to sight your rifle at 100 yards to achieve MPBR if you do not have access to a longer sight-in facility.

So sighted, you can eliminate some of the classic trajectory concerns, focus more on learning lead and point of impact beyond MPBR, and enjoy the hunt.

You can learn more about the Jack O'Connor Hunting Heritage and Education Center by visiting the website: www

.jack-oconnor.org