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6 April 2020, 10:59 #1Contributing Member
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Reloading For The AR15: The Accuracy Node Detection Technique
I’ve had numerous requests for this over the years, so here‘s an introduction to one of the techniques that I use for developing a handload for the AR15.
Reloading For The AR15: The Accuracy Node Detection Technique
The Accuracy Node Detection Technique (ANDT) is simply a tool to aid handloaders in determining the specific powdercharge that is most likely to deliver the highest level of accuracy (technically precision) from their AR15 with a particular powder and bullet. Those of you familiar with Creighton Audette’s Incremental Load Development Method (Ladder Test) or Dan Newberry’s Optimal Charge Weight method may recognize that the Accuracy Node Detection Technique is based on the same underlying principle as those methods, however, the ANDT is a more refined and statistically meaningful approach to finding an accuracy node.
The ANDT uses larger effective sample sizes for greater confidence and utilizes the mean radius to determine the radial dispersion of composite shotgroups to obtain a more exact identification of the accuracy node of a given barrel/bullet/powder combination. For those of you not familiar with the mean radius, I’ll be explaining that shortly.
The ANDT is not dependent upon “interpreting” the data like other methods; the data is what it is and the results speak for themselves. However, keep in mind that 60% of a load’s accuracy comes from the bullet. If you’re not using a quality bullet that your barrel “likes,” no amount of powdercharge testing is going to result in a load that produces subMOA 10shot groups from you AR15.
A chronograph is not needed for the ANDT. If you’re only interested in longrange shooting, this is not the article you‘re looking for. I developed the ANDT for workingup short range (200 yards and in) matchgrade handloads that consistently produce subMOA 10shot groups from my matchgrade AR15s.
As I mentioned above, the ANDT utilizes the mean radius to accurately and precisely determine the radial dispersion of the composite shotgroups that we will develop using the ANDT. The demonstration below shows how to manually calculate the mean radius, but modern ballistic programs make this task far easier.
A PRIMER ON THE MEAN RADIUS
The mean radius is a method of measurement of the radial dispersion of shotgroups that takes into account every shot in the group. It provides a more useful analysis of the consistency of ammunition and firearms than the commonly used method of extreme spread.
Mean radius as defined in Hatcher's Notebook “is the average distance of all the shots from the center of the group. It is usually about one third the group diameter (extreme spread)” for 10shot groups. (The ratio is actually closer to 3.2 times the mean radius = the extreme spread, for 10shot groups, depending on the sample size and the morphology of the particular groups sampled.)
To obtain the mean radius of a shotgroup, measure the heights of all shots above the lowest shot in the group. Average these measurements. The result is the height of the center of the group above the lowest shot. Then in the same way, get the horizontal distance of the center from the shot farthest to the left. These two measurements will locate the group center. Now measure the distance of each shot from this center. The average of these measures is the mean radius.
Once you get the hang of measuring groups using the mean radius it becomes very simple to do. While being very simple to do, it is also very time consuming. Modern software programs such as RSI Shooting Lab and On Target make determining the mean radius a snap.
The picture below is a screen capture from RSI Shooting Lab. The red cross is the center of the group (a little high and right of the aiming point). The long red line shows the two shots forming the extreme spread or group size. The yellow line from the red cross to one of the shots is a radius. Measure all the radii and take the average to obtain the mean radius.
Mean Radius Demonstration
Let’s say you fired a 5shot group from 100 yards and the resulting target looks like this. (The Xring measures 1.5” and the 10ring measures 3.5”.)
The extreme spread of the group measures 2.83”, but we want to find the mean radius (or average group radius.) In order to find the mean radius we must first find the center of the group. By “eyeballing” the target most people would see that the group is centered to the left of the “Xring” and probably a little high, but we need to find the exact location of the center of the group.
Locating the Center of the Group
The first step in finding the center of the group is to find the lowest shot of the group and draw a horizontal line through the center of that shot.
Next, find the leftmost shot of the group and draw a vertical line through the center of that shot.
Now measure the distance from the horizontal line to the other four shots of the group that are above that line. Add those numbers together and divide by the total number of shots in the group (5).
2.50” + 1.03” + 2.01” + 1.30” = 6.84”
Divide by 5 to get 1.37”. This number is the elevation component of the center of the group.
Next we need to find the windage component of the center of the group. From the vertical line, measure the distance to the other four shots of the group that are to the right of the line. Add those numbers together and again divide by the total number of shots in the group (5).
1.76” + 2.54” + 0.45” + 1.19” = 5.94”
Divide by 5 to get 1.19” This is the windage component of the center of the group.
Finding the windage and elevation components of the center of the group is the most difficult part of this process. Once that is done the rest of the process is a piece of cake.
Using the windage and elevation components, locate the position on the target that is 1.37” (elevation component) above the horizontal line and 1.19” (windage component) to the right of the vertical line. This location is the center of the group!
Determining the Mean Radius
Now that we have located the position of the center of the group, the first step in determining the mean radius is to measure the distance from the center of the group to the center of one of the shots. This line is a single “radius”.
Now measure the distance from the center of the group to the center of each of the rest of the shots in the group. Add the measurements of all the radii together and then divide by the total number of shots in the group (5).
0.85” + 1.35” + 1.38” + 0.84” + 1.61” = 6.03”
Divide by 5 to get 1.21”. This is the mean radius (or average group radius) of the group!
Using the mean radius measurement to scribe a circle around the center of the group gives you a graphic representation of the mean radius. This shows the average accuracy of all the shots in the group. This demonstrates why the mean radius is much more useful than the extreme spread in evaluating the radial dispersion of our rifles and ammunition.
The table below will give you an idea of the relationship between the mean radius and extreme spread for 10shot groups.
Here are some interesting quotes from old issues of American Rifleman on the subject:
“Mean radius is the mean distance of bullet impacts from center of the test group. It is used in government ammunition acceptance because it takes account of every shot and comes close to maximizing the test information. While there is no exact relationship between this measure and the simpler and more convenient group diameter, the 10shot group diameter averages slightly over 3 times the mean radius.”
"These examples illustrate the sensitiveness of the extreme spread to number of shots in the group. Indeed, as the table indicates, the measures made to only the outside shots of the group, e.g. the extreme spread, are very sensitive to number of shots, while the measures made to all the shots, e.g. the mean radius are far less so. It may be added that the latter measures are also less variable in their representation of the group; they are more efficient. This explains why the target testing of U.S. military rifle ammunition is by mean radius."
Now that you understand the mean radius, we can delve into some particulars of the Accuracy Node Detection Technique. For the example that I’ll be presenting in this article, I was looking for an accuracy node for the Hornady 53 grain VMAX (#22265) when charged with VihtaVuori N133 powder and fired from my 223 Remington 24” Krieger barreled AR15. But first, a word from our sponsor:
WARNING!
Reloading is an inherently dangerous activity. The information provided here is for educational purposes only. It is not intended to be used for the actual loading of ammunition by the reader. No warranty, guarantee or assurance that these loads are safe is stated, suggested or implied nor should any be inferred. Usage of this information for the actual loading of ammunition may result in malfunctions, damage and destruction of property and grave injury or death to beings human in nature or otherwise. Don't even view this information in the presence of children or small animals.
53 VMAX on the left . . .
VihtaVuori N133
24” Krieger barrel . . .
In my experience with handloading for precision AR15s that are chambered for 223 Remington, I’ve found that an accuracy node can often be identified somewhere within a 1.5 grain spread of powdercharges; not always, but often. For this example of the ANDT, I used powdercharge weights ranging from 23.0 grains to 24.4 grains of VihtaVuori N133. The criteria that I used for this particular example of the ANDT was as follows:
I wanted to find an accuracy node that was within the limits of the precision that the factory powder measure on my Dillon XL650 was capable of throwing. When using a shortcut extruded powder like VihtaVuori N133, the Dillon factory powder measure will throw the majority of the charges within plus or minus 0.1 grains of the nominal charge. However, a significant amount of the time, the Dillon factory powder measure will throw a charge that is plus or minus 0.2 grains of the nominal charge. Therefore, I needed an accuracy node that could maintain the desired level of precision on the target, throughout a plus or minus 0.2 grain powdercharge. Hence, I used an increment of 0.2 grains in the powdercharges used for this example of the ANDT. The following eight powdercharges were used:
Powdercharge #1  23.0 grains
Powdercharge #2  23.2 grains
Powdercharge #3  23.4 grains
Powdercharge #4  23.6 grains
Powdercharge #5  23.8 grains
Powdercharge #6  24.0 grains
Powdercharge #7  24.2 grains
Powdercharge #8  24.4 grains
The powdercharges tested in this example of the ANDT were all dispensed (not thrown) to the exact tenth of a grain using a Pact Digital powder dispenser and scale and verified on a GemPro250 scale. The handloads were otherwise loaded on the Dillon XL650. Five rounds of each of the eight powdercharges were loaded, for a total of 40 rounds. Next, some actual shooting.
All shooting for this example of the ANDT was conducted from my benchrest setup at a distance of 100 yards. The 24" Krieger barrel was freefloated. The freefloat handguards of the rifle rested in a Sinclair Windage Benchrest, while the stock of the rifle rested in a Protektor bunnyear rear bag. Sighting was accomplished via a Leupold VARIX III set at 25X magnification and adjusted to be parallaxfree at 100 yards. A mirage shade was attached to the top of the freefloat handguard. Wind conditions on the shooting range were continuously monitored using a Wind Probe. The lower receiver housed a Geissele HighSpeed National Match trigger. The setup was very similar to that pictured below.
The Wind Probe
The shooting portion of the Accuracy Node Detection Technique should be conducted at a distance of 100 yards to help mitigate environmental variables. When conducting the shooting for the ANDT, you absolutely most monitor the wind conditions on the range and attempt to fire every shot under the same wind conditions. For those who think that the wind “doesn’t matter” at 100 yards, consider the following example.
With the right ammunition, my precision AR15s are capable of producing consistent subMOA 10shot groups at 100 yards. Now, let’s say that you are a quarter of the way through the firing of 40 rounds for the ANDT. The wind has been calm up to this point. For your next shot, you miss the fact that a 7 mph wind is now blowing from 3 o’clock. The wind dies down again and you continue shooting. Three quarters of the way through testing, you miss another 7 mph wind that has kicked up, only this time it’s coming from 9 o’clock. By not paying attention to the wind, your test results have now been increased by more than 1.5 MOA, simply due to wind deflection. So yes Virginia, the wind does indeed matter at the distance of 100 yards.
In order for the ANDT to produce valid data, you positively have to be able to properly execute the fundamentals of marksmanship 40 times (or more) in a row. When conducting the shooting portion of the ANDT, all rounds should be fired in a roundrobin manner. This means that you fire one shot only at the first target using a round from the first powdercharge, then you fire one shot only at the second target using a round from the second powdercharge and soon until you’ve fired one round at each of the eight targets. You repeat this process until all 40 rounds have been fired. Unless you had some collateral damage, you should now have eight targets all of which have five shots on them from each of the eight different powdercharges being tested. Now it’s time to head home and analyze the data.
A sample target . . .
Here’s were things get interesting. We now have eight targets and each of those targets has a 5shot group on them that was fired in a roundrobin manner. Each 5shot group was fired using rounds loaded with the same powdercharge. For the ANDT, we are not interested in the extreme spreads of those 5shot groups. We’re not even interested in the mean radius of the individual 5shot groups. What we are interested in, is the mean radius of the composite groups that we’re going to form from the eight individual targets.
To form the composite groups, we first have to individually enter the eight, 5shot targets into the computer program. In this case, I’ll be using RSI Shooting Lab. Once the individual targets are entered into the program, we can then use the program to overlay targets to form composite groups.
Do you remember earlier in this article that I stated what my criteria was for this example of the ANDT? I stated that “I needed an accuracy node that could maintain the desired level of precision on the target, throughout a plus or minus 0.2 grain powdercharge. Hence, I used an increment of 0.2 grains in the powdercharges used for this example of the ANDT.” Since each target contains a 5shot group from the same powdercharge, and each incremental powdercharge was increased by 0.2 grains, we’re going to overlay the targets in sequences of three, to form 15shot composite groups, giving us more effective sample sizes, particularly when used in conjunction with the mean radius. That way our 15shot composite groups will be made up of rounds fired from three different powdercharges; the nominal powdercharge (the one in the middle), a powdercharge that is 0.2 grains less than the nominal powdercharge and a powdercharge that is 0.2 grains more than the nominal powdercharge.
For example, the first 15shot composite group will be formed by overlaying target #1, target #2 and target #3. The second 15shot composite group will be formed by overlaying target #2, target #3 and target #4. We’ll continue this sequence of overlaying the individual targets to form 15shot composite groups until all eight of the individual targets have been used to form six, 15shot composite groups. These six, 15shot composite groups are what we have been working towards. The mean radii of these 15shot composite groups are going to tell us where our accuracy node lies; no interpreting” required, no soliciting of “opinions” needed.
Here’s a visual demonstration of the process of overlaying the individual 5shot targets, in sequences of three, to form the 15shot composite groups.
Here’s target #6 by itself.
Here’s target #6 and target #7 overlayed on each other.
And targets #6, #7 and #8 overlayed on each other.
NOTE: It is extremely important when overlaying targets for the ANDT that the targets are overlayed on the centers of the targets, not on the centers of the groups. When overlaying targets using RSI Shooting Lab, the program gives you two options: “Aim Point” and “Center”. The “Aim Point” option is the correct option to overlay the targets on the centers of the targets. The “Center” option will overlay the targets on the centers of the groups.
Before we get to the final results of this example of the ANDT, I’d like to use target #6 (that was shown above) to demonstrate the futility of using the extreme spreads of individual 5shot groups to locate an accuracy node. The 5shot group on target #6 has an extreme spread of 0.42”. That’s a sub½ MOA group at 100 yards fired from a semiautomatic AR15 (and remember too that these groups were fired roundrobin). If I were using the extreme spreads of 5shot groups to locate an accuracy node, I would think I had found it with target #6 . . . . but I would be mistaken, as we shall now see.
Here are the mean radii of the six, 15shot composite groups. The nominal powdercharge of the 15shot composite group that has the smallest mean radius is the accuracy node for the 53 grain VMAX load charged with VihtaVuori N133 and fired from my 223 Remington 24” Krieger barreled AR15.
And there’s the accuracy node! Targets #2, #3 and #4 overlayed on each other formed a 15shot composite group that has a mean radius of 0.18” at 100 yards.
Now that we’ve identified the accuracy node at 23.4 grains of VihtaVuori N133, let’s take a look at what this load can do when fired from my Krieger barreled AR15 at a distance of 100 yards.
Hornady 53 Grain VMAX
3shot group: .086 MOA
5shot group: .317 MOA
10shot group: .533 MOA
Here are some pics of groups from other handloads that I developed for my Krieger barreled AR14 using the Accuracy Node Detection Technique.
Sierra 55 Grain BlitzKing
3shot group: .088 MOA
5shot group: .206 MOA
10shot group: .439 MOA
62 Grain Berger HP
5shot group: .28 MOA
10shot group: .483 MOA
…Last edited by Molon; 13 February 2021 at 10:29.

6 April 2020, 12:10 #2
Thank you!
Bump and a bookmark for a more thorough read later.

7 April 2020, 15:46 #3
Thank you, Molon. Super helpful. Love the mean radius discussion particularly. Make sense.
Quick question: when you shoot for testing groups, how do you overcome the first cold bore shot? Any procedure you recommend besides shooting a first round with other ammo?

8 April 2020, 06:59 #4
As always, great information presented in a manner that is easily understandable. Thanks.
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8 April 2020, 12:41 #5
We appreciate the hours you put into these thorough tests so we can digest them quickly. Thanks so much.
“ When I comes to modern politics, I think the inverse of Hanlon's Razor applies...In other words, "Never attribute to stupidity that which is adequately explained by malice."  Kerplode