Red Dot Hacks: Battle Sight Zero [Maximum Point-Blank Range]
2/15/2019Primary Arms Staff2/18/2019 7:12 amwww.primaryarms.com
Zeroing A Red Dot: Using a Battle Sight Zero
You’ve read our article on the most important zeroing hack. Your new optic is mounted properly, you remembered to bring a screwdriver to make your adjustments, you’ve brought some quality ammo that will hold a tight group if you do your part, and you’ve got sight-in targets with an inch grid printed on them.
Now, at what distance should you place that target to achieve the most versatile performance, and why? Opinions differ on where exactly your rifle’s zero should be located. Some folks advocate measuring the longest possible engagement distance in your residence and zeroing at that distance for a home defense firearm, reasoning that bullet impact at any further distance is not a realistic possibility. Others simply zero at the distance where they plan to shoot the most —- a shooter with a maximum of 50 yards available at the local indoor range might choose a 50 yard zero, and a shooter with a 100-yard outdoor range might choose 100 yards, simply out of convenience more than anything else. However, a more sophisticated approach can maximize the capability of your red dot equipped firearm –- the Maximum Point-Blank Range (MPBR) zero.
Understanding MPBR requires a deeper discussion about what exactly is happening when we sight-in a scope. To start with, imagine a perfectly straight green laser beam coming out of an AR-15 barrel, and a red laser beam coming perfectly through the centerline of the mounted red dot. If both of those laser beams are pointed straight ahead, parallel to each other, they will always be about 2.7 inches apart. This distance is called “height over bore”. If you have seen those viral videos of hunters accidentally shooting holes through their pickup trucks, you know not to forget about height over bore! Of course, we are shooting bullets, not lasers. While our line of sight through the optic is perfectly straight, our bullet is affected by gravity and begins to drop as soon as it leaves the barrel.
If the scope’s line of sight and the barrel are perfectly parallel, point of aim and point of impact will never converge at all. The bullet will start at the height-over-bore distance below the optic and drop from there, opening the gap up further. Therefore, zeroing your setup involves tilting the barrel’s centerline muzzle high a very small amount so that the bullet’s trajectory carries it upwards until it crosses the scope’s centerline. That’s where point of aim meets point of impact —- the distance where the bullet crosses your point of aim is the range of your firearm’s current zero. Because gravity is always pushing down constantly on the bullet, it will cross that point of aim once on the way up (sometimes called “initial intersection”), and again a second time at a further distance after gravity pushes it back down (sometimes called “true zero”).
This phenomenon is expressed as two numbers separated by a slash. For example, the US Army uses a 25/300 meter “battle sight zero” where initial intersection occurs at 25 meters and true zero happens at 300 meters. The US Marine Corps uses a 36/300 yard scheme, and many American shooting instructors and schools have adopted a 50/200 yard scheme. The practical difference lies in the maximum range where you can expect to achieve a critial hit on a target of a certain size without shifting your point of aim at all. Being able to simply put your red dot on your target and achieve a hit is your maximum point-blank range.
How MPBR Plays Out In The Real World
The military MPBR schemes attempt to assure a hit anywhere on an enemy combatant from head to belt buckle, so that dictates the size of their acceptable target area. Using the Marine Corps 36/300 yard standard as an example (sorry Army friends), if our Marine aims his red dot squarely in the middle of a steel torso target at all ranges, the round will impact slightly below his point of aim if the steel torso is closer than 36 yards. At 36 yards the bullet’s impact will coincide perfectly with point of aim, as the bullet climbs upwards. At 100 yards, the bullet would impact about 3.5 inches high, and at 200 yards, about 4.6 inches high because the angle between barrel and scope is still causing the bullet to climb. This area is called the “mid-range rise” or, more technically, “maximum ordinate” -— at 200 yards aiming at the chest area of the steel torso will result in a hit on the head or neck area. After 200 yards gravity begins to take its toll and the bullet falls back down, so at 250 yards bullet impact is 3 inches high, and at 300 yards, true zero occurs with the bullet hitting exactly where the red dot is aimed. At 350 yards the bullet has dropped almost 5 inches from the point of aim, and at 400 yards the bullet is nearly a foot below the point of aim, which falls outside the acceptable margin for a hit. Therefore, to the military’s standards, the Maximum Point Blank Range of a 36/300 yard zero is somewhere between 350 and 400 yards.
The 50/200 MPBR zero trades a lesser maximum total effective range for better precision within that effective range. When using this standard, a bullet's total deviation from the point of aim is less than 3 inches all the way from 50 yards through 250 yards. For hunters who need to keep their shots within a 10” circle to ensure a humane harvest of the animal, this is a great setup out to 250 yards. Beyond that, the bullet falls dramatically. At 300 yards it is 7” below the point of aim and at 350 yards it is more than a foot below the point of aim. If you want to stretch a red dot past 250 or 300 yards, 50/200 is not the right strategy for you. On the other hand, if you don’t want to worry about your shot sailing high of a target located in the mid-range rise, it works great. The 50/200 MPBR is very popular among civilian and law enforcement shooters in the USA who need to take precise shots up close, but don’t anticipate taking a legally justified shot beyond 200 yards with their hunting, patrol, or home defense rifles.
Keep in mind that all these examples thus far are working with military standard rifles and ammunition, namely AR-15 carbines shooting 5.56 NATO ammo. Change barrel lengths or change bullet weights, and those numbers no longer apply. What about changing calibers or rifle types entirely? Don’t worry. You can easily apply the MPBR concept to your red dot equipped rifle by employing a ballistic calculator like StrelokPro, Ballistic AE, or JBM Ballistics. Using these calculators, you can input data from your exact rifle setup, define the target size you want to stay within, and create your own custom sight-in solution. For example, we plugged in the data for a standard 16” AK-47 shooting inexpensive 7.62x39 ammunition. Original military doctrine on AK iron sights called for sight-in using the “P” setting on the iron sight at just 18 meters (19.5 yards), giving the trajectory an arc of about 7” total out to 300 meters. The rear leaf sight could then be set to various ranges to compensate for bullet drop. But what about a red dot-equipped AK that will only use one 2 MOA dot as the constant point of aim? For this common setup, a 50 yard zero results in a minimal mid-range rise of just 1.5” at 150 yards, but by 250 yards the relatively fat and slow 7.62 round has already dropped more than a foot. Sighting in at 36 yards, Marine Corps style, results in more mid-range rise but a still useable drop of just 9” at 250 yards. Or you could mimic the original military standard and sight in your red dot at 18 meters—it all depends on your priorities!
These ballistic calculator apps are better than ever and a lot of fun to play with, so don’t be afraid to dive in and calculate the Maximum Point-Blank Range scheme that works best for you and your set-up. It can be done! And the benefit you get is a better understanding of your rifle's capability, making you more effective as a shooter.
The bottom line is, MPBR allows you to get the most versatility out of your red dot sight. So put a little more thought and effort into your sight-in session on the range and you’ll reap the rewards!
As always, if you have any questions about this article, the wide variety of red dots offered by Primary Arms, or any other products we carry, we encourage you to call us at 713-344-9600 or send an email to email@example.com.