In this article I want to consider realistic grouping in relation to a point blank zero. The basic way I understand that point blank zero (PBZ) works is that the zero is set so that when the rifle is fired with the point of aim centered on the target, the maximum ordinate (the top of the arc) of the trajectory would be at the top of whatever size target you have in mind. The distance at which the bullet drops to the bottom of the target dimension is the maximum point blank range. The actual zero range, somewhere in between, is just an afterthought.
The basic idea for the point blank zero is to remove as many things that the shooter needs to pay attention to as possible through taking into account the amount of leeway the size of the target allows. Another way of saying it, relating to the analogy of tolerances, is that as the target gets larger, and the sighting system adjusted accordingly, some of the rest of the tolerances of the shooting system can be loosened. Looser tolerances means that the process is easier and faster. The guiding requirement is that the shooter hits the target. Where the bullet strikes the target is not so critical. Simply aim at the target center and still hit some part of it. The primary variables that are typically taken into account when optimizing a point blank zero are the size of the target and the trajectory of the round as it relates to the point of aim.
Take for example the AR I’ve been using lately and the 4” target I’m working with. I hit a snag right off the bat because I have to divide the 4” into a top half and bottom half, each 2” tall. The sight height on the AR at this moment is 2.6”, which is greater than 2”. So in this case when the rifle is aimed at the target center and fired, the bullet doesn’t rise to the bottom of the target until about 10 yards.
Red crosshairs indicate point of aim. Black dot indicates point of impact at close range (inside 10 yards or so). Neither this, nor any or the other illustrations that follow are precise. They are relatively close approximations intended to convey the idea.
Therefore, technically a point blank zero is not possible with a 4” target and that AR. I would have to be doing something wrong to accidentally get a hit if the target were closer than 10 yards and I actually aimed at it. Getting a hit because two or more things I did were mistakes does not sit well with me. A 5.2” target would theoretically be just fine for a point blank zero.
If I just figure that I’ll adjust my point of aim to adjust for mechanical offset for close range shots and still roll with my attempt at the PBZ with the AR and 4” target, and I set the initial intersection for 50 yards (which is a common zero, known as the “Improved Battlesight Zero”), using Federal XM193 from the Noveske 16” barrel, and an average muzzle velocity of 3041, the maximum ordinate listed in my ballistics program (Shooter) is 1.9” from 125 yards to 150 yards. The actual zero turns out to be 220. The bullet drops to 1.8” below the point of aim at 250 yards. 255 turns out to be too far, with 2.2” below the point of aim. To sum up, the 220 yard zero theoretically keeps my bullet within the target out to 250 yards, which would be the maximum point blank range, again assuming we ignore that first 10 yards.
The big problem with this illustration, which I reused from a 2011 article, is that the muzzle is quite incorrectly depicted as being above the initial intersection, which with the theoretical AR discussed in the article, would would actually occur at approximately 50 yards (the initial intersection).
The ballistics program does not take the phenomenon of grouping into account. It assumes a single point of impact for a given zero at a given distance. Because we often let computers do our thinking for us, it can also lull the shooter into thinking the same thing.
If I did a good job at setting my zero, the center of my group would be at my exact point of aim at the zero distance. That means at the maximum ordinate and at the maximum point blank range (at the extreme ends of the tolerances), about half of my rounds will miss the target.
Black impacts are simulated at the maximum ordinate. Green rounds are at actual zero. Red rounds are at maximum point blank range.
What if I adjust the point blank zero according to how the rifle shoots this ammo? I have a 10 round 1.5 MOA group with this ammo. It would probably be safe to extrapolate that over the course of many more rounds it might keep them inside 2 minutes. I could take that data into account when setting my PBZ. Since my theoretical group in this instance is 2 MOA, and half of it is going over the target at the maximum ordinate, I need to build in a 1 MOA buffer. This turns the whole affair into a trial and error game with the ballistics program. I found that if I set my initial intersection to 67 yards, the maximum ordinate is 0.7” from 115 to 125 yards. One MOA at 125 (1.047*1.25) is about 1.3”, which makes it look like this could work. The zero range turns out to be 170 yards. Unfortunately, in order to keep a 1 MOA buffer at the bottom of the target, it means that 175 is my maximum point blank range. That’s still a pretty good deal for such a small target (approximately 2.2 MOA at that range).
Come to think of it now, I shot that 1.5 MOA group from a rest. What if I had to use a field position? The only thing close to that is a 10 shot group fired from the open leg sitting position, which was approximately 2.8 MOA. I have no long term data to extrapolate that out, so let’s just take it at face value. Setting the initial intersection at 82 yards would keep me in the target on the high end, which occurs at 125 yards. The actual zero distance would be 140 yards. At 140 yards I have pretty much no extra leeway in my seated group to take it any farther and still keep them inside the target (a target that is perfectly shaped, easily visible, at a known distance, and completely stationary).
So what on paper initially looked like an ability to shoot out to 250 yards without worrying at all about elevation, turned out to be a dicey proposition at 140 yards from what is actually quite a steady position. If I had to use something less stable, such as kneeling or offhand, my own limitations as a shooter are going to keep me way inside that distance anyway. That would also mess up all the numbers I ran to get that point blank zero to work, and I would probably be sending some of the rounds over the target even within a range that I would be good with if I had stuck with the 100 yard zero.
Aside from the two variables that are typically considered, target size and trajectory, it’s crucial to consider the capabilities of the shooting system (including the shooter) to maintain a small enough grouping to stay inside the target. Limiting the discussion to a 4” target does make for an extreme example, but is also a good way to illustrate that there are some practical limitations to the point blank zero concept. If I were shooting at a larger target such as a big game vital zone I would have more leeway to make the point blank zero concept actually do something for me. The larger target would allow for looser tolerances in the shooting process. I don’t know if it would reasonably allow for my typical standing group size though.
The point of this installment of the shooter tolerance stack is that there are times when you can leave a lot of the details out of your mind and still get a hit. An extremely steady shooting position will provide a cushion for compromise or mistakes in other areas, as will a large target. Carrying that concept over to small targets and unstable positions, such as standing, will not yield consistent results. If conditions force you to fire from your feet, the burden on you to account for other variables is greatly increased.
I’ll round out my thoughts on this subject in the next installment.