If you had to rely on one zero, what would it be?
For a long time, the mantra has been that the 50 / 200 yard zero (which are not truly the same mind you) is the most useful zero for the AR15 platform as the flat trajectory allows near point of aim / point of impact to nearly 200 yards. It’s vanilla advice, and vanilla pretty much suits anyone, but each would individual would benefit from exploring the benefits of the different zero’s as it relates to his or her rifle.
Many of you might have heard of the maximum point blank range method of zeroing. It encompasses a single zero where the projectile is set to a zero that keeps it afloat in a defined vertical zone / arch to match the largest diameter of a target that you want to shoot. As an example, if the vital zone I want to shoot is six inches in diameter, I would zero the bullet path to arch 3 inches high and when it drops 3 inches below my line of site… that is the end of my maximum point blank range.
Hunters have used this method for a while, but we have to tighten it up a bit for use in a defensive setting. If we stay inside the vital zone, we would be OK in theory… but with bullet accuracy and shooter error factored in, we have to adjust the MPBR to float in a defined space despite inherent accuracy issues present in the shooter / rifle combo.
So the 50/200 +- yard zero is a great starting point, and depending on the projectile weight and barrel length, it will come up 1-1.5 inches of maximum ordinate (aka the bullets apex) and then dip below 1-1.5 inches where the MPBR would end. Since we count the total rise and fall, that gives us 2 to 3 inches of vertical resolution where the bullet will rise and fall in a 2 to 3 inch line.
That’s a pretty tight zero. Is it too tight? We can almost hit small game with that zero, but is a defensive rifle set up for small game or human sized targets? What’s the smallest point we might aim at on a human target? Let’s start by expanding upon why we want to ditch the 50/200 zero mantra and see how things go with a 4 inch diameter MPBR:
Point. Click. Hit.
If I am shooting a 20 inch AR15 with XM193 and a maximum bullet rise of 3 inches high, then I just extended my MPBR to 299 yards where at that point the bullet would dip 3 inches below the line of sight. That has effectively lengthened our resolution to 6 inches of rise and fall. That easily remains in the space occupied by a human head out to 300 yards! Remember, this sighting method is target defined and shooter-rifle-projectile unique. So in a self defense setup, we need to maximize the distance where we will hit our target without shot correction.
If the target is inside our zone… we want the rifle to be a simple point and click interface. Bam. Hit. We want the highest probability of hitting a head, torso, or half exposed limb. Point. Click. Hit. We want to maximize hang time to increase the probability our bullet will hit the target at unknown distances. Click. Boom. Hit. So why not 4.5 inches up and down if the average head is 9 inches tall? Why not a 3.5 inch zero for a 7 inch diameter vital zone? It all depends on your expectations of precision and how accurately and repeatability you can hold your rifle on the center of your intended target.
Goal: increase the probability that the projectile will intersect the target without calculations.
Factors: Rifle accuracy, shooter accuracy, target size, environment.
I would think that a target, once engaged, would be hiding and would present a small target for the shooter. Three inches of rise and fall (for a total of 6 inches of vital zone) gives us a great starting point for hitting hiding, peeking, or partially exposed targets. Rifle and shooter accuracy will play a role in deciding the final zero as we might be inside of the vital zone (say a headshot) where the bullet should just nick the target at the bullets apex… but if our ammo / shooter combo is shooting with 2MOA accuracy the INACCURACY could send the bullet another inch higher and that results in a missed shot.
So that’s why we need a little bit of wiggle room. 6 inches of vertical resolution should give us a good likelihood of hitting the target with a weapon capable of 1.5-2 minutes of accuracy… but if you can dial the accuracy down to 1 MOA with a match bullet, the MPBR system will deliver more consistent hits and we will get to that in a minute.
Six inches of total vertical resolution seems like a logical choice for a MPBR setup and extends us past our 50 yard / 200 yard zero a bit and is still a fine resolution for engaging defensive targets. I can’t think of anything I would shoot at that would be missed because my trajectory apex is three inches high. Look at your knee. It likely has a targetable area of at least six inches. Torso? This resolution is easily inside the vital zone of a center of mass hold.
So next I whipped up a basic chart to get an idea of what zero at 100 yards I would need to get a MPBR which targets a 6 inch radius. This is actually a very simple process as you would zero at 100 and then round up (in clicks) to the closest 1/2 value. If you have a 1/4 minute or 1/2 minute scope, red dot, or other optic you are all set.
XM193 and Mk 262: Sight in at 100 yards X high in blue for a bullet apex of 3 inches; the MPBR is in red for each barrel and projectile. The XM855 would benefit at similar zeros as it will be in between the MPBR of the 77 grain and 55 grain ammo.
Barrel Length 10.5 14.5 16 18 20
xm193 2.6 2.4 2.5 2.2 2.0
MPBR 262 281 289 300 303
Mk 262 2.8 2.6 2.6 2.5 2.4
MPBR 242 261 270 279 284
So you can see there is a bit of difference in the MPBR between rifle, carbine, and bullet. At one end we have the longer point blank range of the 55 grain XM193 and at the other end the sexy new hotness of the 77 Mk 262 at the other. If you were to build a field gun for targets from 0-300 yards, your probability of hitting any targets without aiming correction (provided it fits within your targets diameter) will increase by the use of military spec 55 grain ammo out of a 20 inch gun. Another benefit of this zero is reduction of necessary holds: Out of a 20 inch gun with a red dot and XM193, two holds would get you hits out to 400 yards, COM for targets out to 300 and the head for targets out to 400.
This method of sighting in gives you some excellent hit probability. If the target is near, a dead hold will hit anything your aiming at. If you estimate the target is at 300-400 yards, holding on the head will ensure rounds drop into the mid torso. If you were incorrect and your target was closer than you estimated, your projectile would still likely hit the head.
It appears that M193 or quality defensive 5.56 equivalent such as Hornady 55gr TAP… would make the best choice because of the 55 grain projectiles flat trajectory, and having the extra distance where we could still hit small targets helps… but when wind is thrown into the equation the M193 makes it more difficult to hit the small targets at the end of its MPBR. So back to the heavier match bullets, what benefit do they offer? Increased accuracy and increased wind resistance.
Accuracy, Wind, and MPBR:
Typically M193 is considered a 1.5 to 2 MOA bullet and Mk 262 is considered a match sub MOA bullet. Think of each bullet as a probability to hit an area based on its accuracy and construction. Groupings are the defining measure of whether a more aggressive MPBR would work. If your gun cannot keep groups inside the defined zone, then the distance you have chosen won’t work.
Ideally you should start with a accurate, rifle length free floated barrel, and use the best ammunition available to you. Bonus points if you reload. In the below example, I tightened the acceptable MPBR to 2 inches apex for a total resolution of +-4 inches witch is another useful form of MPBR. I like 3 inches, but the example best illustrates accuracy and wind drift of two common projectiles. The hold is on the nose or dead center of the face. The large red circles represent your accuracy and points of impact (both high and at the very end of your MPBR) with a 1.5 minute 55 grain projectile, and a 1 MOA or sub MOA projectile in the Mk262 example. The blue circles represent a mild 5 mph wind drift of each projectile.
As the above illustration demonstrates, the heavier match ammo still shoots flat enough to give us 250 yards of headshot range out of a rifle, while being more accurate and fighting wind better. At 400 yards the bullet dips a bit further down the torso than M193, but that’s still a hit.
Wrapping Up:
So ultimately, your shooting goals should play into the rifle and its associated setup and upgrades. A red dot or BUIS set up with a MPBR zero will assist you in hitting targets of opportunity and at any distance inside your MPBR box. Since we cannot effectively range estimate every shot on fleeting targets, best practice would be to utilize MPBR to ensure hits by maximizing bullet flight hang time as it relates to the size of our target. This method may be off-putting to those with a ACOG or similar rifle optic since it negates the benefit of the bullet drop stadia.
Overall, it seems that a 1.5-2 inch high zero at 100 yards will get you very close to the 3 inch maximum apex for several loadings. As we deviate away from the 20 inch gun, maintaining a 2 inch 100 yard zero “rule of thumb” across all barrel length and bullet choices will slightly reduce your MPBR less than what the chart says as you go with heavier bullets and shorter barrels. Best practice appears to be a heavy match bullet out of an accurate rifle length platform. This should increase your success in hitting targets without much correction.
The true value of this setup is that it helps free you from thinking about your target in hundreds of yards, instead it allows you to estimate either near or far. Point. Click. Hit.
Integrating this into my setup… I foresee finalizing my Razor HD II for two projectiles: 55 Grain Hornady TAP for general purpose, XM193 for practice, and I am moving to Hornady 75 gr HPBT for longer range work. The Hornady is considerably cheaper than the 69 grain SMK I have been shooting, and switching back and forth between the two loadings is as simple as uncapping the dial and rotating in a few clicks of elevation and wind-age. I think these loadings can cover all shooting I can ever realistically do. I really like this method as it increases the probability of intersecting the target at x range inside my MPBR zone. It’s important to get away from the 100-200-300 yard paradigm and examine a zero which allows a more fluid approach to hitting a target at unknown ranges.
The goal of a marksman is to study the platform and integrate the best practice findings into his or her shooting. I think it is safe to say that for work inside of 400 yards, the 77 grain Mk 262 offers the best probability of hitting the target if shot out of a rifle length system, and its excellent characteristics make it best for environmental conditions. XM193 makes a good substitute as your holds would be the same, but wind may knock you off target without some very light wind correction. This method has now replaced my “chest, head, hat” methodology which I was using with the RDS.
It’s time to test my new method at the range. Vacation week cannot come soon enough!
Parent article: http://www.thenewrifleman.com/maximum-bullet-flight-getting-the-most-out-of-ar15-zero/
-----------------------------------------------------------------------------------------------------------
-----------------------------------------------------------------------------------------------------------
No comments:
Post a Comment