Group Size Measuring.

Group Size Measuring.

How do I correctly measure the size of my rifle's group?

How do I correctly measure the size of my rifle's group?

How to measure the group size. Measure your group size using the center-to-center method. Use a digital or dial caliper or a precision ruler to measure the distance from the outside edges of the two bullet holes farthest apart on your target, as shown above. 

In this case the outside measurement for the three shots is .642 inches. Then subtract one bullet diameter (as determined by the ammunition you are firing) from the measurement. Since this group was fired with a rifle chambered in .308 Winchester, we subtract .308 from .642 to give us a center-to-center group size of .334 inches.

.308 Winchester
.308 Winchester is used in this example.

The target itself. Remember that paper or cardboard is not a very good medium to get a really accurate measurement from. While, for demonstration purposes we measured our three-shot group to 1/1000th of an inch, in the real world you’ll be doing well if you can get an accurate measurement to 1/100th of an inch. You can feel good rounding off a 1/1000ths inch measurement to 1/100th of an inch. You might also try a target printed on crisp, starched short fiber heavy paper or card stock to produce a clean, round bullet hole.

Grouping Measurement
This group was fired with a rifle chambered in .308 Winchester, so we subtract .308 from .642 to give us a center-to-center group size of .334 inches.

Converting Your Group Size to MOA (Minute of Angle or Arc). The term MOA - Minute of Angle (sometimes called Minute of Arc) can be confusing, but it’s really quite simple. A full circle, for purposes of things like navigation or angular measurement, is divided into 360 degrees. (You’ll see this marked on the edge of your magnetic compass or a drafting protractor.) In turn, each one of those 360 degrees can be further divided into 60 minutes, each of which is 1/60th of one degree.

The minute of angle becomes a very handy measurement for rifle shooters because 1 MOA very closely matches 1” at a distance of 100 yards. (It’s 1.047” at 100 yards if you want to be precise.) Thus rifle shooters often call a 1” group fired at 100 yards distance a “1-MOA group.”

So in our example above, the size of the three-shot group fired at 100 yards is .334 inches, which we can round off to .33 inches and that’s very close to 1/3 MOA.

Diagram for understanding MOA vs Distance
MOA (Minute of Angle) values are approximate. 1 MOA is actually 1.047" at 100 yards. At practical shooting distances the fraction is not a problem.

MOA increases with distance. Remember, while MOA remains constant the group sizes increases with distance

The same 1 MOA group will naturally become larger in direct proportion to the increase in firing distance. Assuming we discount any environmental factors such as a crosswind, a 1 MOA group measuring 1” at 100 yards will increase to 2” at 200 yards, 3” at 300 yards, 6” at 600 yards and so on. The level of accuracy doesn’t change but the group’s size simply increases in direct proportion to the firing distance. 

This also means when you dial in corrections on your scope or sights, you need to make sure you’re taking your firing distance into account. If your scope adjustments move the point of bullet impact ¼ MOA per click, you’ll need four clicks to move your point of impact 1” at 100 yards, two clicks to move the point of impact 1” at 200 yards and just a single click to move the point of bullet impact 1” at 400 yards. 

Here is an excellent video to help you better understand MOA at the NSSF website.

A note on ideal accuracy vs. real world accurately.  Remember that when you cherry pick a super small group it’s good for the ego but it isn’t really representative of the average accuracy of the gun or load. Organizations like the NRA typically fire five 5-shot groups when they are doing accuracy testing to get a better feel for what one can expect from a given firearm. Four five-shot groups is also a pretty good standard for reliable data, and that’s typically one full box of rifle ammo.

Also remember that environmental conditions play a big factor in accuracy. If you do your accuracy testing and get a perfect zero in the middle of a blistering Texas summer, things will probably change a bit when you’re on a bone chilling late season elk hunt high in the Colorado Rockies. Wind can also play havoc with accuracy. If the crosswind is blowing so hard that you just saw Dorothy and Toto fly away, it’s probably not a good day to test for accuracy. 

Finally, keep rifle accuracy in perspective. It’s easy to become obsessed with shaving an extra 1/8th of an inch off your average group size by trying out countless different types of ammo or spending hours at the loading bench playing with bullet seating depth and primer selection. But if your scope or your current level of marksmanship will only allow you hold 3 or 4 MOA in a given situation, you’re better off finding a good load that provides a dependable and acceptable level of accuracy, and then spending your time and money on the range improving your shooting and your wind reading skills. 

As with most things in life, there is no substitute for practice. Marksmanship is a perishable skill that needs constant reinforcement. Make time for a range session on a regular basis. Besides, the worst day on the range usually beats the best day at work.

Winchester has long been known for producing rifles designed to acheive extreme accuracy. Great accuracy is the sum of a great deal of variables. Many of them involve you, the shooter. Other variables, like ammunition quality and optics, become a choice. Not to mention, the shooter must be able to shoot well. And that usually takes years of practice as well as a little bit of instruction. It is important to start with a precision rifle that is capable of exceptional accuracy, then build the package. 

FAQ article copyright Winchester Repeating Arms, September 2016. Written by Winchester Repeating Arms staff writer, and avid rifle target shooter, Scott Engen.