I previously wrote about the relationship between a bullet’s ballistic coefficient, velocity (and therefore barrel length), and choice of zeroing distance along with associated effects on shot patterns at various ranges. The Swiss took advantage of this relationship with their S4G system (Sniping- 4th Generation).
As Colorado Pete pointed out in the comments last year, this concept has existed for a while as something called the point blank zero (PBZ). In short, this is the zero distance for a given rifle and load combination that keeps your shots within a desired radius while using the same point of aim. For example, consider a vital zone size of eight inches in some large game animal. The PBZ would be the zero in which the bullet would impact no more than four inches above or below point of aim, which is the center of the vital zone. A larger vital zone means a more generous point blank zero; a smaller vital zone means a restricted one.
How does this concept work?
Recall that a bullet does not fly in a straight line. Rather, it flies in an arc. The characteristics of this arc are the result of the aerodynamic shape of the projectile (the ballistic coefficient, or BC) and the velocity it is traveling. The BC and velocity are consistent from shot to shot. The only thing affecting the shape of the arc is the angle at which the projectile is launched- which you control by adjusting the elevation turret. In a practical sense, this alters where the bullet initially crosses your line of sight, how high it will reach (also known as the maximum ordinate), and where it intersects your line of sight again as it falls.
The end goal of a PBZ is to choose a maximum ordinate and a point below the line of sight that encompasses the acceptable target area. If your shot falls anywhere within this imaginary circle, then it is considered a good hit.
JBM Ballistics has a fantastic calculator that takes a lot of the guess work out of this for you. All you need to do is feed it your ballistic data and the desired “vital zone” radius. For this chart, I am using Sierra 69gr SMK fired at 2750 fps (as I recently measured on the musket). My desired vital zone radius is six inches (meaning a 12 inch total vital zone). Notice that the chart tells me exactly what the calculated PBZ should be for my desired 12″ zone, and what distance that point blank range (PBR) is good for. It also tells me the range of maximum ordinate (161 yards, about 6″ above line of sight).
According to the calculator, the best point blank zero to maintain a 12” target is 291 yards. The calculator predicts that the maximum range I can use this zero is about 341 yards. If I go back into the calculator again using the same ballistic data, and make my zero 291 yards, then you can see how the data shakes out.
Of course, zeroing at exactly 291 yards it not terribly practical, but you can use it as a guideline. Increasing my zero to 300 yards gives me effectively the same result, with a drop of 6.2 inches at 350 yards. Using this data, it’s easy to see why the Army and Marines were drawn to the 300 meter zero (I know, meters are not the same as yards, but the data works out pretty close).
A challenging aspect of this type of zero is remembering to account for the maximum ordinate. With this zero, the bullet will be about 6 inches above the line of sight at 161 yards. If you are aiming for a small target (such as a small gong, or a headshot), then you are liable to miss high if you simply aim for the middle of the target. PBZ works best for center-mass aiming. If your target is smaller, then you need to adjust your radius and zero to compensate. This is the reason for the ubiquitous 50/200 zero, since it supposedly keeps the bullet +/- 2 inches over line of sight until about 250 yards. Note that the 50/200 is notional and based on military load data and velocities, your actual zero may vary (mine is closer to a 30/200).
Alternatively, the A-zone of an IPSC target is eight inches tall. JBM tells me that a 250 yard PBZ works best for that size, keeping me +/- 4 inches out to about 300 yards.
Applying PBZ to Different Reticle Designs
The above concept works well for your average duplex reticle found on millions of hunting rifles. But what about other reticle designs? You can combine PBZ with various reticle shapes to determine more aiming points.
For instance, my Trijicon TR24G has a nice glowing triangle sitting atop a post. However, the heavy post prevents me from having usable holdover positions beyond the bottom edge of the triangle. According to Trijicon, this triangle, at 4x magnification, is 4.2 MOA tall. Since it is second focal plane, it is 12.8 MOA at 1x, and 8.4 MOA at 2x magnification.
JBM does not produce a nice easy PBZ based on minutes of angle, but it does still produce MOA drop values. You will have to input various zero distances until you arrive at an acceptable solution. If I wanted to keep my impacts to about 4.2 MOA above or below the point of the triangle, then it turns out about a 275 yard zero works best. Again, rounding up 300 yard zero with the tip of the triangle means that the bottom of the triangle is at about 410 yards, and that is probably an easier zero to choose, since it keeps consistency with the previous PBZ.
The different MOA subtensions of the reticle at lower magnifications can also be put to use. If I zoom down to 2x, the bottom of the triangle is now 8.4 MOA from the tip, which correlates to a 500 yard aim point. At 1x, that same point now gives about a 600 yard aim point. That is, of course, assuming you can even see the target.
This method works reasonably well for any optic that does not have some form of range markings (either as a BDC as in an ACOG or my ELCAN, or as hash marks in an MRAD reticle).
A 25 yard zero actually works out very well for this combination of rifle, load, rifle, and optic. A 25 yard zero keeps me +/- 3 inches out to 250 yards. The bottom of the triangle correlates to about a 350 yard aiming point. At 2x, the bottom of the triangle is about 450 yards, and 550 yards at 1x. Not bad for such a simple triangle.
In the end, you have to imagine two opposing ends of a line. On one end is speed, and on the other is precision. Which is more important to you for the most likely shot you are going to take? For hunting or self-defense at reasonably close distances (inside 200 yards) on fairly large target zones, a PBZ method is very effective. For fast-paced timed competition, the PBZ is workable with a well-chosen zero. If you need to hit small targets at longer ranges, then you might need to shift more towards precision-oriented zeroing methods and optics.