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The Optimal AR Rifle Zero: How to find your zero distance for optimal trajectory

August 15, 2017

 

What distance do you zero your AR? Most shooters pick a nice round zero distance, such as 25 yards, 50 yards, 100 yards, 200 yards. But why do you select that distance? Because it’s where the yard markers are on the range? Because you always did it that way? Because some internet Guru said so? Or is it because you understand that the zero distance determines the flight path of your bullet and you selected a distance to optimally match your rifle and ammo creating a bullet flight path that makes hitting targets at all distances very easy?

 

You should have a precise zero distance for your specific rifle and ammunition combination and it is highly unlikely that your zero distance is a round number. Would you rather have easy to remember holds or easy to remember zero distance?

 

Don’t have time to read? Click for The 7 steps to finding the Optimal Zero Distance

 

If you don’t know how to Zero click here (Video)

 

Bullet Flight Path is the Foundation of all Shooting

 

If you spend a lot of money on your AR, if you train to be more proficient with your AR and especially if you carry an AR for your job, then you must put some thought into your zero and bullet flight path. If you don’t, you are undermining your performance significantly.

 

The distance you zero your rifle at determines your flight path, that is undisputed. What is disputed is the best distance to zero your rifle? This is disputed widely on the internet. In most discussions the participants argue about the best zero distance, but they ignore the underlying rifle/ammo combo they are arguing over. All those conversations are a waste of time unless you are talking about a very specific rifle/ammo, which they are not. The real question should be, what is the best flight path for my AR? Once you solve that, then finding the zero distance is easy.

  

We define the optimal flight path as one that is designed to maximize the distance from the muzzle before we must apply any hold at all and to minimize the number and complexity of holds to memorize from 0-500 yards.

 

This concept is not new and we did not come up with it, but it seems to be completely lost on most AR shooters. Both the USMC and the US Army manuals seek to achieve a similar flight path.

 

“An appropriate battlesight zero allows the firer to accurately engage targets out to a set distance without an adjusted aiming point..” – US Army Training Circular 3-22.9, May 2016.

 

“the Service Rifle’s BZO setting will enable engagement of point targets from 0 to 300 meters/yards in a no-wind condition.” – US Marine Corps Reference Publication 3-01A, October 2012.

 

They have standardized weapons and ammo so they can achieve this on a large scale, but other factors such as range time, ammunition allocation and shooter skill come into play. Many civilian shooters and especially Veterans get confused and think the USMC or Army zero distances are good for their private firearms. Our definition of the Optimal Flight Path is essentially the same as the concept of Maximum Point Blank Range (MPBR) which may have originated with Jack O’Conner. MPBR is mostly discussed in the hunting community. While it isn’t as widely known in the AR / Tactical community, MPBR still applies.  We cannot wrap our heads around why they chose this name for this concept as “Point Blank” is quite literally a single point in space and it is right at the muzzle, so there cannot be a maximum point blank range. We also have not found anyone who clearly shows an AR shooter how to attain this flight path for his individual rifle/ammo and that is what we intend to do here. We have found some really good explanations of the concept. (Video - Jeff Gonzales, Backcountry Chronicles Article) Our description of the optimal flight path is very similar to the theory of the 50/200 yard zero, but because all rifle/ammo combos are unique the 50/200 doesn’t fit all situations. We also found that the 50/200 yard zero does not optimize the Maximum Ordinate (the maximum height of the bullet flight path over the line of sight) and its relation to a relative target size. We have used the 50/200 zero plenty and it is a good zero, it is just not optimal.

It is important to note that all of these methods seek the same thing, to maximize the immediate distance from the muzzle before the shooter must apply holds on the target.

 

Click for Definition of External Ballistics Terms at Primary & Secondary

 

Why is it so important to make the holds simple and easy to remember?

Applying holds takes time and some estimation, so the likelihood of a miss increases anytime you apply a hold. In life and death situations things are complex and stressful. Living targets do not expose themselves for long periods of time and they do not stand still very long so it is critical to make hits on the target as quickly as possible. Most marksmanship practice happens on static ranges where the targets do not move. This gives the shooter a false sense of time to hit a target at a specific range. Even if the shooter uses a timer to create the effect of limited exposure, he still knows what distance the target is at so he has time to calculate his holds in his head before the buzzer goes off. In a real world situation the targets are moving, fleeting and appear at unknown distances and at unknown times so the shooter must simplify all his actions to be able to make hits. This video shows quick transitions from target to target at varying ranges with solid hits and minimal calculations.

 

 

Your rifle is unique from all other rifles, treat it that way. 

 

The AR platform is highly customizable, a number of the options you can choose affect the ballistics of your flight path. You can choose a barrel with different length, twist rate, profile, construction, you can choose sights or mounts with varying sight heights and you can choose gas system size and type. When you buy your rifle you are selecting these features you just may not have realized it. Now consider the different type of ammunition you can choose and the variables that affect flight path, powder type, amount of powder, primer type and bullet weight which affect the velocity of the round. Then think of the ballistic coefficient of that bullet which basically is a measurement of how efficiently the bullet flows through the air. If you are careful when purchasing your bullets, you take note of weight and velocity. Do you know what barrel length or twist rate the manufacturer uses to test the velocity? If you don’t know that then the velocity on the box is useless.

Not all variables are listed here, but it is important to highlight the main variables and that they exist. We are not considering the wear, use, maintenance of each individual rifle that can create differences in identical rifles. With so many choices to make it is fair to say that each rifle, especially the customized rifles, are unique.

 

To illustrate the point of uniqueness in rifle/ammo combinations, let’s take an imaginary Special Operations unit that has brand new standardized rifles and optic setups, but the shooters can select the type of “Standardized” ammo they carry. One shooter choses M855 Green-tip 62-grain, another carries M995 Black-tip 52-grain AP for the penetration and yet another chooses Mk 262 Mod 0/1 77-grain Open tip match. Each of these rounds have unique and different velocity and weight creating unique flight paths even though all other variables are the same. In this example everything is exactly the same except the bullet weight and velocity. Each of these rounds have different flight paths. This extreme example shows that your specific rifle/ammo combination should be treated as it is unique and if you change your ammo then that should be treated as it has its own ballistic characteristics.

 

Setting Constants 

 

To calculate the Optimal Flight Path we must determine some constants. We must select a measurement, a target size and a max distance.

 

Measurements: Imperial (yards and inches)

Target size: 6 inches

Max Distance: 500 yards

 

We measure everything using the Imperial System because we are Americans, so yards and inches. If you use meters in determining distance you must make the calculations to yards. For example 500 yards is equal to 457 meters, they are not the same. We select a 6” circle because it fits into the frontal view of an average human head and when shooting at larger targets such as the vital zone of big game (9 inches) it gives us a healthy margin for error. We will show you how the target size determines the Maximum Ordinate and starting with larger targets steepens the flight path out at distance. We select 500 yards as a distance, because we need to limit the range in our discussion and that is what the USMC uses for qualifications for all Marines. Once you understand the optimal flight path, it is easy to extend this distance beyond 500 yards or change the size of the desired target.

 

How to find the optimal flight path for my unique rifle?

 

Understanding the Tube

Keeping in mind that our goal is to maximize the distance from the muzzle before we must apply any hold at all and to minimize the number and complexity of holds to memorize from 0-500 yards.

 

This image is from http://www.shootingtimes.com/ammo/ammunition_oconnors_practical_field_ballistics_013111/

 

If we can imagine shooting down a tube where the center of that tube is the line of sight through the scope. The bullet exits the muzzle below the line of sight approximately 2.5-2.7”. This equates to most standard sight heights for AR’s. If your sight height is greater than 3” because you have a Tavor or a sight on top of an M4 carry handle, then you can increase the target size to accommodate (roughly 8”) or you can ignore the fact that your bullet will be below the ”tube” for the first few feet of the bullet flight. We want the bullet to stay in that tube and we want that tube to be as long as possible. Knowing that the flight of the bullet is an arc which starts roughly at the bottom of the tube we must create an arc that does not go higher than 3” above the centerline of the tube. Translating this to the rifle, the highest point of the bullet flight, aka Maximum Ordinate must be 3” (radius of our constant target size) above the line of sight. Then we must calculate, when a bullet is shot with a Maximum Ordinate of 3”, how far will it travel before it falls 3” below the line of sight. We will do this with a Ballistic Calculator. The calculator will tell us how far our unique rifle/ammo combo will go before it leaves the tube. This distance is the maximum distance we can shoot to while still holding center on the target. For example if my tube length is 300 yards then I can place my reticle center on the target as long as it is closer than 300 yards I will hit the 6” target keeping in mind that the round may be up to 3” high or 3” low of the point of aim. The creation of the tube allows the shooter to ignore the variations that happen in the first 300 yards of flight.

 

Ballistic Calculator 

 

Today we can get highly accurate Ballistic Calculators in the form of Applications on our smartphones. Most all high quality ballistic calculators have the same features and are relatively cheap from a few dollars to $20. Most precision shooters use these apps, but AR shooters tend to ignore them even though they are a very useful tool to understand the flight path of your bullet. The line of logic goes like this, if you like to hit things you shoot at, you need to know your bullet flight path and the variables affecting that. The best way to understand that flight path is with a ballistic calculator, so go get one. It is worth every penny, but if your cheap, then borrow one or do the math by hand.

 

We chose the Shooter (Ballistic Calculator) from Kennedy Development Group LLC. We have no affiliation with them, it’s just the calculator we chose and it works great.  If you don’t know how to use a Ballistic Calculator start here with this tutorial video. (ADD LINK) Enter all aspects of your rifle and ammunition, this needs to be done once and make sure the information is as accurate as possible. With that said, the biggest pitfall is the bullet velocity. I highly recommend using a chronograph for an accurate velocity. The velocity of the round is usually displayed on the ammo box or manufacturer website, however most manufacturers use a 20” barrel to test velocity and a large number of AR’s do not have a 20” barrel. If you use the provided velocity instead of a chronograph remember the rule of thumb for every inch of barrel loss, bullet velocity drops by about 50 FPS. The velocity of your bullet as a very significant factor in your bullet flight path. The other significant factor is the zero distance. In this App, it is in the Ammunition Edit section. Start your zero distance for 50 yards.

 

 

 

Once all your data is put into your Ballistic Calculator, run a solution graph for 200 yards. Enter 200 into the distance and press calculate.

 

Focus on the Path (in) column, look for the highest positive number. It will likely be around 150 yards and somewhere around 1-3”. The image to the left shows the highest positive number is 2.1”. We want that number to be 3”.

 

To change the Maximum Ordinate you need to slide the Zero distance in the ammunition edit screen. To increase the Maximum Ordinate slide the zero distance back from 50 yards.

 

 

 

 

If you need to Decrease the Maximum Ordinate increase the distance from the 50 yard start point.

In this app changing the zero distance requires the shooter to flip through multiple screens and then recalculate the solution. This is a tedious process, but only needs to be done once per Rifle/Ammo combination and is critical to optimizing your flight path.

 

In this example we found that a Zero distance of 43 yards gives us a Maximum Ordinate of 3.2” which is close enough to 3”. We ran a solution for 300 yards to demonstrate, notice the arrow on the highest positive number which is the Maximum Ordinate.

 

 Now that you have identified the unique zero distance that will give you an optimal flight path, go to the range and precisely place your target at that Zero distance. Any errors in distance will drastically affect the actual bullet path, so measure the distance very carefully. A laser rangefinder is recommended.

 

Now that you are Zeroed go back to your calculator and let's find the precise distance at which your flight path will fall below 3”. In the image above you will see that at 300 yards the bullet is -3.2” from the line of sight. You need to drill down to find the exact yard line simply run a solution for specific yard lines until you find exactly where the path is 3” below the line of sight.

 

 

 

 

For the Optimal flight path in this example 298 yards is close enough to 300 yards so we will simply call it 300 yards. This is the length of the theoretical tube that we are trying to establish. It is also the furthest distance where we can hold center on a 6” circle and still have theoretical hits. For any 6” target from 0-300 yards we will simply hold center and send a round.

 

Now we have to figure out some holds out to our defined maximum range. In your ballistic calculator run a solution for 500 Yards.

 

 

From this table I need to figure out holds from 300 - 500 yards. To do so I simply look at 400 yards to find a drop of 16.4”. We know that it is easier and faster to remember round numbers instead of decimals so we round this to 15”. Rounding can be tricky, because you are balancing an accurate hold with ease of recalling that hold, make your best judgement call and go with it.

For 500 yards the bullet drop is 38.9” which we will round to 40”. The holds break down to this.

 

0-300 yards = Hold Center on 6” target

400 yards = Hold 15” over (holding top of the head will give a high thoracic hit.)

500 yards = Hold 40” over (estimate 40” or use reticle)

 

As with any Zero, the holds must be confirmed in real life. Go to the range and actually shoot targets out to 500 yards. This step is critical and will flesh out any mistakes you may have made in the process. If you have conflicting information when comparing your ballistic calculator output and the impact of the actual round shot. The actual data from shooting trumps calculator data every time.  If you need to adjust your holds, do so based on real world data.

 

 

Quickly calculating the holds for any range

 

How do we know where to shoot if we have a target between the distances I memorized? You must find the nearest holds closer and further than the target distance, calculate the overall drop between the two and then estimate the relation of the distance to the drop and add it back to the closer hold. For example: If my target is at 450 yards I know my drop at 400 = 15” and 500 = 40”, the bullet drops 25” in between those two holds. 450 is halfway between so it will only drop half of the 25” or 12.5”. I add that 12.5” to 15” and I hold 27.5” over the target. Let’s compare this to the chart above and you will see that the calculator reads 26.3” at 450 yards. The 1.2” difference in the calculation shouldn’t matter much. Because the calculation method is simple and reduced the number and complexity of the holds we need to memorize, we use the calculation method for any distance between our memorized holds.

Keep in mind that at 300 yards the bullet is already 3” below the line of sight so the difference between 300 and 400 is 12” not 15”.

 

The shooter can memorize as many additional holds as he likes, but the intent is to keep it simple to start. As the shooter gets stressed, he should be able to refer to something simple as a few holds. To further finetune the use of the Optimal flight path, the shooter should know the yardage for the important points along the bullet's flight.   

Zero distance (43 yards in example) as that is point of aim point of impact

Maximum Ordinate (150 yards in example) is 3” above line of sight

Repeat zero where the bullet crossed the line of sight for the second time (258 yards in example).

Furthest Distance without a hold (298 yards in example)

 

Understanding the Theoretical Flight Path from the Ballistic Calculator

 

The Ballistic Calculator puts out a very precise number which can only be as accurate as the input it receives. Although the ballistic calculator will accept atmospheric data they can change faster than your ability to calculate them. Let’s look specifically at wind data, it is highly unlikely that the wind along the entire flight path will be constant and exactly what you entered into the calculator. The variance in the wind will cause the bullet to hit a different spot than the calculator provided. Furthermore, there are accuracy thresholds for your ammunition, your rifle and even you the shooter. No one is perfect, no rifle is perfect and no ammo is perfect, so this will create deviances in the actual flight path. It is impossible to factor all of these things into a calculation so the calculator provides the best answer it can. This is why we highlight that the output from the calculator is the Theoretical Center of most normal flight path variances.

For Example if the inherent accuracy of your system (ammo, rifle & shooter) is 3 MOA you will shoot all bullets within a 3” circle at 100 yards. In the example provided above the theoretical center of that 3” circle will be 3” above the line of sight at Maximum Ordinate. That means that some of those rounds will be 1.5” above the theoretical center which puts some of those rounds outside of the “Tube”. This is the main reason why we choose a 6” target not 8” (profile of head) or 9” (vital zone).

 

Why put so much effort into optimizing your AR zero?

 

This is a concept of optimizing your flight path to make hits on targets with a minimal amount of time. The key word is optimizing. Having an optimized flight path is critical to those who train to shoot living things with a gun. Live targets don’t stand still and human targets usually fight back one must be able to acquire the target and know the proper hold very quickly to be successful. You can either put the time in planning out and simplifying your holds ahead of time or you will have apply complicated holds in the limited time you have to hit your target.  

 

7 Simple steps to Optimizing your bullet flight path - Each Step is explained in detail in the paragraphs above.

 

 

   

 

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