Ammunition testing – how should you test for precision?
Hexagon Ammunition’s Paul Bradley describes the way many manufacturers will test ammunition precision and how a seller might be able to utilise some of these methods themselves.
The first important factor we must consider are the variables involved. Testing ammunition is a scientific experiment after all. We must understand which variables are at play and how we can best remove any that are confounding. Let us imagine the testing generally conducted by a shooter when trying out a new batch of ammunition. They go to a range with their rifle and shoot some groups. They then measure the groups. Most often they will select the nicest looking group and suggest that is what the ammunition is capable of. However, there are a huge number of confounding variables involved which will negate any validity this test had.
What condition was the rifle in? What it windy? What temperature was the barrel at? What surface were you shooting from? Are you a consistent high-level shooter? And many more besides…
In an ideal world we would reduce all confounding variables because otherwise we are simply measuring the system as a whole on a given day – The system being rifle, optic, mount, operator and ammunition.
Let’s look at how these variables can be removed so we can focus on the ammunition itself. In the industry We would conduct any precision firing at an indoor range which has a controlled environment. Of course, this removes wind but also ensures that the ambient temperature of the environment, rifle and ammunition remain the same for all tests carried out. Precision testing will usually be conducted via two very different systems. The first being a fixed barrel and receiver that can be fired remotely. This eradicates the human error variable but does not really mimic what one can expect from a factory rifle. The proof barrels are very heavy and fixed in position. They are also carefully cleaned a monitored for wear. In essence they represent the ammunition performance as a “best case scenario” and provide solid data. This data is only valuable in comparison to other proof barrel firings and of course has little value to a civilian shooter who would not own such a rig. But it is about as near to a sound scientific test of precision as one can get as a manufacturer.
Multiple groups will be shot and a mean average taken. It is vital you do not “cherry pick” good groups and discard less favourable ones. Any dispersion can work in two ways, bullets can deviate and make a group worse but they can also make it better, that is down to pure chance. Taking a mean average reduces the percentage of lucky or unlucky groups. The more groups you shoot to draw an average from, the less apparent that error will become.
How many rounds should the group be? This tends to vary somewhat between manufacturers. The more rounds in a group the more you heat the barrel up. Heating the barrel can cause a loss of precision and thus becomes a confounding variable. For this reason, I am a fan of multiple three round groups, giving the rifle time to cool to ambient between each string of firing. These groups can then be measured and even overlaid to show a group of hundreds of rounds if one so wished but without the heat build up which would no doubt skew results.
Many manufacturers will also test ammunition precision through a number of popular rifles. This gives a better indication of the results a customer might expect to see. The customer can also take some tips from this testing to replicate it and reduce confounding variables to a minimum. You can see in the picture from my colleagues at Norma USA they recommend using a heavy sled to hold the rifle. They would commonly use a bench type mount similar to something seen in an F Class competition. This will go a long way to reducing shooter error. We all like to imagine our shooting skills are 100% perfect but this is rarely the case!
If you are stuck with an outdoor range then limit the distance you are shooting at. Testing at 1000 yards may sound cool but in fact you have only magnified wind effecting the projectiles. I have lost count of the number of times I have tried to explain this to potential customers who want to assess precision at extreme distance. You end up assessing the shooters ability to read wind and not much else!
Try to conduct your testing on a calm day with settled weather and fairly average temperatures. Then one can shoot multiple 3 round groups as described and draw a mean average from the results. This is as near to factory testing as a person can get without access to environmentally controlled ranges and fixed accuracy rigs.
Last but not least how does one measure these groups?
Extreme spread
The images show how one measures extreme spread. The measurement is taken between the two shots which are furthest apart in the group. The measurement is taken from centre to centre (or you can measure edge to edge and subtract the calibre size). This has long been the generally accepted standard for measuring precision in the shooting world. It is easy to understand and quickly achieved with a ruler or Vernier caliper. As extreme spread is the commonly accepted measurement we are often asked to supply it when detailing ammunition performance. It does however have a major drawback. If one shot is erratic due to shooter error or environmental effects then the precision could seem poor when viewed numerically. In the left-hand group, you can clearly see this. The main group would be sub MOA and perfectly acceptable to a customer. Unfortunately, one erratic flier more than doubles the group size which on paper would lead a customer to believe the ammunition was of poor quality.
On the right-hand side, we can see a group with much larger general dispersion which would be deemed poor. However, it shares the same measurement in extreme spread. In this instance it is clear that extreme spread was not the best choice when trying to determine which group was the “best” as on paper they are the same. When viewing the target itself the right-hand group would be clearly be preferable.
Mean radius
This method of group measurement is not commonly used by shooters but is becoming increasingly popular in industry. Mean radius involves plotting the group centre and then measuring the distance of each separate bullet strike from the centre. This can be done by drawing a vertical and horizontal axis on the extreme left and bottom of the group. One can then measure the distance of the shots in relation to the axis and conclude a mean measurement (group centre). Then it is just a case of measuring the distance of each shot from the calculated centre and producing a mean of those figures. This is obviously time consuming but can be achieved with the help of a smart phone and an app (several available). We tend to use this when assessing our ammunition as it tells a story about every shot in the group rather than the two most erratic.
Sellers can also utilise these factors. If you are running a demo on a rifle or ammunition then all the above factors reply. I have seen demos quickly go downhill because of shooter error or a badly chosen venue. Always consider your variables and try as much as is reasonable to reduce any that will be confounding.
We’d love to hear your thoughts on this topic, e-mail your views over to editorial@fieldsportspress.com
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