Measuring Group Size – Brownell’s Shooting Tips

Measuring Group Size
Measuring Group Size

Des Moines, Iowa – -( Editors Note: Gunsmith Barry Shell shows us how to measure target shooting group size to gauge you or your firearms performance at the gun range.

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.

When you go, one thing you need to be armed with is the knowledge of how to measure shot groups. Group size is the main measurement of our shooting abilities because it allows us to judge our personal performance or our gun’s capabilities.

The easiest and most commonly used method of measuring shot groups is the “center-to-center” method.

This is 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 (#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.

  1. Find the Bullet Diameter – 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.

  2. Measure Your Bullet Hole Group – Once you know bullet size, measure your group from the edges of the bullet holes farthest apart on your target, as shown:

    The calipers read 1.818″.

  3. Subtract Bullet Diameter from Bullet Hole Group Size – 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 the example formula and another example:

Bullet Diameter – Bullet Hole Group Size = Group Size or .308 – 1.818 = 1.51

Measuring Group Size
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.

Founded in 1939, Brownells is an Iowa-based, family-owned company that supplies more than 75,000 firearms parts, accessories, reloading components, gunsmithing tools, and ammunition to armorers, gunsmiths, and shooters worldwide. In addition to their industry leading 100% lifetime guarantee on EVERY product sold, their staff of veteran Gun Techs are available to assist customers with any need – free of charge. There are no minimum order sizes or fees. To place an order, or for more information, call 800-741-0015 or or visit

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Why can’t you measure distance between the holes directly?

Larry Miller

Frank, that would work if there was only one bullet size(diameter).
Since you are interested in distance from center to center for all bullet sizes, the formula in the article is necessary.


If you have two shots in one ragged whole and one flier, it’s hard to find the insde of the furthest hole. The author was trying to make it as simple as possible Smaller groups are more difficult to measure.

Captain Bob

That works well so long as you don’t have a small group [which I always have.. :)] as you can’t always tell where the inner edges are but the outer edges are always obvious.


I am new to the shooting sport. I can’t wait until I have your problem. Sounds like you need to move the target out another 100 yards.


Or… you could measure from the outer edge of one bullet hole to the inner edge of another hole and you won’t have to do any math.


That’s how I do it…