Most precision rifle shooters and reloaders like to measure the size of their groups to determine the level of precision attainable with a given rifle and ammunition combination. If you are new to this sport, group size can become confusing at times, with groups measured in inches and minutes of an angle (MOA- sometimes called a minute of an arc) and from center to center and edge to edge. Hopefully this post will demystify the task for you.
The easiest way to measure a shot group is to simply measure the distance from one outside edge to the other. While this provides data, measuring your group size from center to center allows an accurate comparison of precision between different rifles or different rifles. Keep in mind, the holes on a .30 caliber rifle will always be physically larger than those from a .22 caliber rifle.
To accurately measure the size of your groups, you’ll need a set of calipers (above). These tools typically measure dimensions from 0-6″ within .001″ of an inch! Calipers are configured in a number of different ways, some use a digital readout (above, top), some use a dial (above, bottom) and some use a Vernier scale (not shown). Dial and digital calipers are the most often encountered by reloaders, with digital being easier to use.
This is the sample shot group we are going to measure. Note I placed pertinent information on the top of the target, “6 Creed, 7/30/16, 107 SMK, H4350, 2.750″ OAL” and “41.0” below. This tells me the cartridge, 6 Creedmoor, date, July 30, 2016, bullet, 107 Sierra MatchKing (SMK), powder, Hodgdon H4350, overall length (OAL) of the finish cartridge, 2.750″ and powder charge, 41.0 grains (read the sites disclaimer loads are only considered safe in the test gun). The more information you write on the target the better. In this case, we will be measuring from the outside edges of the most distant shot groups, shown by the yellow lines above.
Since we are measuring from outside edge to outside edge of the shot group, we will need to subtract the diameter of the bullet in order to determine the size of the group center to center. This is where the digital calipers have a big advantage, simply close the calipers on a bullet the same size as the hole and hit the “zero” button. The calipers are now zeroed to the bullet. It is a good practice to lay the zeroed calipers over one of the holes in the target to verify the zero matches the size of the hole in the paper.
Note, when they close, they now show -.243″, the diameter of the bullet.
With the target on a flat surface, simply place the calipers on the target and carefully adjust the jaws until they contact the outside edges of the target. In this case, the group measured .750″ center to center (note the .0005″ appeared when I put the calipers down for the picture, if this was the reading when I took the measurement, I would have rounded up to .751″).
Dial calipers area little different. This is my trusty Lyman dial caliper I purchased new back in 1993! In this case, you still measure from the outside edges of the most distant shots, however, you will need to subtract the bullet diameter. On our sample target above the dial caliper reads .993″. To determine the group size center to center, simply subtract the bullet diameter (.243″) from the reading (.993″-.243″=.750″). In this case, the group size is .750″.
How to convert to MOA
MOA refers to a minute of an angle. I remember when I was a teenager in the pre internet days trying to figure out what Jeff Cooper was talking about when he suggested he wanted a rifle that would shoot under a minute. What took me a while to figure out back then is a click of the mouse away now. Most shooters will refer to 1 MOA as 1 inch at 100 yards (this is sometimes referred to as inch per hundred yards or IPHY), and while this a rough approximation, it is incorrect.
For a moment, let’s take a trip back to school. You’ll recall a circle is divided in 360 degrees. Each one of these degrees is further divided into 60 minutes (a circle is made up of 21,600 minutes). One minute of an angle or 1 MOA (or minute of an arc), refers to this section of the arc of a circle, or 1/60th of a degree. From a given point, this arc will subtend (or cover) 1.047″ at 100 yards, 2.094″ at 200 yards, 3.141″ at 300 yards and so on.
So how do we convert to minute of an angle? Well, that is fairly straight forward. To convert to MOA, you need to know the distance from the target and the group size. Since 1 MOA = 1.047″ at 100 yards, if you target is 100 yards away, simply divide the group size by 1.047″. Using the example above, .750″/1.047″= .716 MOA. You’ll note that the MOA measurement is always slightly smaller than the measurement in inches at 100 yards. See, math is cool!
For distances other than 100 yards, you can do it the easy way or the hard way. The easy way to use a web-based calculator, the harder way it to convert the value on your own.
To make the calculation yourself, you’ll need to calculate how large an MOA is at the given distance, (range in yards/100 yards) x 1.047 MOA. Then divide the size of the group in inches by this value.
For instance, a 2.500″ group at 200 yards. 1 MOA at 200 yards= (200 yards/100 yards) x 1.047 = 2 x 1.047″ =2.094″. Now divide the size of the group, by this value, 2.500″/2.094″= 1.194 MOA.
For shorter distances, such as a 1.000″ group at 50 yards, the same formula works. 1 MOA at 50 yards = (50 yards/100 yards) x 1.047″ = .500 x 1.047″ = .524″. So 1 MOA at 50 yards equals .524 MOA.
If you don’t like the math, stick with the calculator, it is OK!
Got it? Practice time! Let’s measure some groups!
Sample 1:
This is a 5 shot 100 yard group, shot with a 308 Winchester (.308″ diameter Sierra MatchKing) and measured with dial calipers. The distance from outside edge to outside edge of the group is .970″. What is the group size in inches center to center? What is it in MOA?
.970″-.308″= .662″, the size of the group center to center. .662″/1.047″= .632 MOA.
Sample 2
This is a 10 shot, 200 yard group fired with a 6.5 Creedmoor (.264″ 142 grain Sierra MatchKing). The digital calipers were zeroed on a bullet prior to measuring the group. What is the group size? How many MOA is it?
Group size is easy, simply read the calipers, .731″. Note that in the image above, the blade of the caliper moved on the target when I took the picture). MOA? We have to work for that one.
1 MOA at 200 yards= (200 yards/100 yards) x 1.047″ = 2 x 1.047″ = 2.094″
.731″/2.094″= .349 MOA! (That was prone with a bipod, I was clearly having a great day on the range…)
A few parting tips when you measure your groups:
- Check your work. I like to measure each group three times. If you aren’t getting the same value, something is wrong.
- Make sure the target is flat when you hang it and when you measure it. This will allow for a more accurate reading.
- Don’t lie to yourself, the numbers tell the truth. I’ve seen some funny practices over the years. These targets are your feedback, use the information to help you become a better shooter and reloader.
- Be careful with the computer applications that measure group size. They require scaling to correctly measure the shot group in the pictures. I’ve noticed slight discrepancies with them over the years and prefer to do it the old-fashioned way.
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