Let us start by defining the topic.
Maximum Point Blank Range (MPBR): the distance (in yards) a projectile can travel without rising or falling more than a predetermined measurement above or below the point of aim.
Putting that definition into practical terms, the target matters. If you are hunting zombie groundhogs, the acceptable measure above or below the point of aim may only be half an inch.
That will dramatically limit the distance. With the same firearm, if the target is a raccoon, we may be willing to accept a plus or minus of two inches.
Let’s assume we are using our trusty AR-15 for our small-game hunting needs. For the illustration, we will use Hornady 53-grain V-Max Superformance Varmint (3465 fps) factory bullets.
The army of zombie groundhogs have small heads and zombies need headshots, so a one-inch target size will be our calculation.
In order to determine our MPBR, we will need to access a ballistics calculator. Most of the larger ammunition manufactures have one online. I happen to like the Hornady version.
To use this or any calculator, you will need some information.
|Weight of the Projectile||53 Grains|
|Velocity of the Projectile||3465 fps (24″ Barrel)|
|Acceptable Hit Size on the Target||One Inch|
With such a small target, the MPBR is either going to need to be quite close or you will have to pick a likely engagement range and work with that probability.
The very short zero of 20 yards will create a ½” up or down from roughly 10 yards to about 35 yards.
Seeing as you are not very likely to want to get that close to a zombie groundhog, we will look at a more likely (desired) engagement range.
By choosing a practical engagement range of 75 yards, the MPBR has a much larger engagement envelope. Our ½” up and down spread now ranges from about 40 to 175 yards.
This means that if we place the crosshairs squarely on the middle point of the groundhog’s head (and assuming we are a steady shot) we can expect it to become a “good zombie” between those distances.
From 40 to 70 yards, the shot will be a tad low. At 75 yards, that is our first POA/POI convergence.
From 80 to 145 yards, it will be a tad high (less than 0.3”) with the peak of the arc at roughly 110 yards. At 150 yards, we are again dead on.
From there it will begin to drop and pass our acceptable kill zone at just over 175 yards. The math assumes a VERY accurate rifle and a very solid shooter.
A headshot is no longer needed and the target is larger, so we can accept a hit range of plus or minus two inches.
With the much larger acceptable target zone, the MPBR runs from zero to 250 yards. From zero to about 40 yards, the hit will be no more to 1.5” low.
At 50 yards the POA/POI converge. From 50 to 200 yards, the bullet will be slightly high, with the crest at about 135 yards and one-inch high.
At 200 yards, the point of aim and point of impact will merge again. From there, the impact will drop until the bullet is below our acceptable two-inch margin at just past 250 yards.
This means if we put the lateral crosshairs across the shoulders, we will get a debilitating hit from zero to 250 yards. At very close ranges, it will be a high chest hit.
At the zero points, the neck will be impacted. Between the two zeros, a high neck or headshot will happen. A shot further out past the 200-yard zero, will again result in a chest hit.
There is a different way to look at these numbers.
I prefer to confirm zero at longer distances, but for those who do not have access to that distance for zeroing, the same net effect can be done by performing the zero within 50 yards.
The numbers will work in either direction.
I hope this is not taken as the proper way to zero a rifle for precision shooting. The military calls this type of zero, a battle zero or a battle-sight zero.
Snipers do not use this method. They are often attempting to hit a chest (or head) at well beyond 600 yards. Having a zero of plus or minus close enough is NOT what they need to be successful.
However, someone on the battlefield needs to send a bullet downrange with a strong probability of a hit without having to accurately gauge distance is well served using this method.
As the above example illustrates, assuming the shooter is competent and has a precise rifle, from zero to 250 yards a shot can be taken against a vampire raccoon, with no drop calculation needed.
This is also a very useful item for people who are stalk hunting. Let’s assume the quarry is a (non-undead) American Whitetail deer.
The hunter is unlikely to get closer than 25 yards and probably cannot to hold steady enough with an offhand shot past 200 yards.
In our stalk hunt, we are in lowland scrub of Carolina and choose a blunt, lower-velocity round to aid in breaking through light brush.
Our choice is a .30-30 Winchester using 150-grain Speer soft points.
|Weight of the Projectile||150 Grains|
|Velocity of the Projectile||2350 fps (18″ Barrel)|
|Acceptable Hit Size on the Target||Six Inches|
Probably counter-intuitively, the best initial zero would be 50 yards. With this zero, the bullet will be no more than 1.5” low prior to the 50-yard zero.
It would be dead-on at 50 yards, with a slight rise continuing out to just past 75 yards. The projectile would be a tad low at 100 and drop three-inches below the point of aim at 175 yards.
Moving the initial zero to 75 yards makes the drop closer to four-inches at 175 yards, so it is less useful.
A switch to the much slower (2350 fps) 150-grain .30-30 Winchester round drops our useful engagement range to 175 yards on a deer.
We lose 75 yards of engagement despite having an acceptable hit zone of six inches instead of four inches.
Conclusion: Using MPBR to Zero
I hope this helps to illustrate a way of zeroing a rifle for practical use.
This will not provide the most precision, but it will provide a very useful degree of accuracy for shots where speed is as useful as accuracy.
Or another way of saying it, a quick chest hit is much more effective than a precise ear-canal shot that was never taken.
How do you calculate your MPBR? Have you tried this method? Let us know in the comments section below!