Pro Tip: Measuring Group Size

By Robert Farago

Measuring Group Size

Des Moines, Iowa – ( Now that spring is upon us – really, it is! – shooters in the less temperate regions of the U.S. are dusting off their range bags, digging out the shootin’ irons, and heading out to the nearest range to brush up their skills. If you’re looking to improve your shooting, or just keep an accurate record of your and your guns’ performance, you need to know how to measure shot groups. The easiest and most commonly used method of measuring shot groups is the “center-to-center” method . . .

This means simply finding the distance between the centers of the bullet holes farthest apart on the target.

Finding the center-to-center measurement is done best with a caliper. Since we don’t usually require an ultra-precise, ten-thousandths-of-an-inch measurement for group size, an inexpensive caliper like the Lyman dial caliper (Brownells #539-832-212) is more than adequate for the job. If you don’t have a caliper though, a ruler that measures down to at least 1/16th of an inch will work just fine.

To begin finding group sizes, you must first know the diameter of the bullet you’re shooting. If you’re shooting a bullet designated in inches, like the .308 Winchester or .45 ACP, you’re already in business since the caliber is the diameter. If you’re shooting a metric bullet, like a 6mm or 9mm, you’ll need to do a little converting: 1 mm equals approximately 0.039″. So, for example, a 9mm bullet is about 0.35″ in diameter.

Once you know that, measure your group from the edges of the bullet holes farthest apart on your target. In the target at the top of this post, the calipers read 1.818″. This four shot group was made by a .308, so the next step is to subtract the bullet diameter from the group diameter. In this case we have 1.818 minus 0.308, which equals a group size of 1.51″, from the center of each of the farthest holes.

Here’s another example:

Measuring Group Size

In this instance, the calipers measure 0.834″, and the bullet was a .308 again. So we have 0.834 minus 0.308, for a group size of 0.53 inches, center-to-center.

It’s as simple as that! Measuring your groups will help you evaluate your own shooting for the day, but to take the most advantage of group sizes, you should keep track of them over time to see if you’re getting better or if you need to change something – or to track a gun’s accuracy. To do that, you can use a log book, like one of the Brownells Modular Data Books. During each range session, simply write down the group sizes you shoot, along with the firearm and cartridge information, and your distance from the target.

The Brownells data book even has a place to draw a sketch of the group as it appears on your target so you can track points of impact versus points of aim in various conditions. Collected over time, this data can help you measure your performance, the accuracy of your firearms and cartridges, the life of your barrels, and it can even help you diagnose problems – a topic for a future article.

As the saying goes, knowledge is power. When you start measuring your groups and keeping track of them, you’re collecting the data you need to start squeezing every ounce of performance from yourself and your firearms.


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