Extreme Spread vs Mean Radius: How to Measure a Rifle Group

Two shooters can fire the same group and report different numbers. Usually neither is lying; they are measuring different things.

Why group size is a measurement problem

"It shoots half-MOA" is one of the most repeated sentences in precision shooting, and one of the least informative. It rarely says how many shots were in the group, how many groups were fired, whether the best one is being quoted, or how the size was measured. Those details matter more than the number itself.

This article sticks to the measurement. It covers the two common ways of describing a group, extreme spread and mean radius, plus the directional measures that matter at distance. It does not give accuracy figures for any rifle or ammunition, because those depend on the rifle, the ammunition lot, the shooter and the day. What it gives you is a way to make your own numbers mean something.

Extreme spread: the traditional number

Extreme spread (ES) is the distance between the centers of the two shots that are farthest apart. It is the number most people mean when they say "group size," and it is what benchrest and most informal reporting use.

How to measure it

1. Find the two holes that are farthest apart. On a round group it is obvious; on a stringy group, check a couple of pairs.
2. With calipers, measure from the outside edge of one hole to the outside edge of the other.
3. Subtract one bullet diameter. For a .308-caliber bullet, subtract 0.308 inch; for a 6.5mm bullet, subtract 0.264 inch. The result is center to center.

Torn paper makes holes look slightly smaller or larger than the bullet, which is why some shooters measure inside edge to inside edge and add one diameter instead. Either method is fine if you use it consistently.

What extreme spread is good for

It is fast, needs no software and matches how most people talk. For a quick zero check or a rough comparison between two very different loads, it does the job.

What it hides

Extreme spread depends on only two shots. In a five-shot group, three holes have no influence on the number at all. One wide shot can double the result; a lucky group can halve it. And because each added shot is another chance to land a new outlier, extreme spread tends to grow as the shot count grows, which makes three-shot and ten-shot groups impossible to compare directly.

Mean radius: using every shot

Mean radius (MR) is the average distance of each shot from the center of the group. Because every hole contributes, it moves around much less from one group to the next than extreme spread does. That stability is why analysts, ballisticians and many serious competitors prefer it for comparisons.

How to calculate it

1. Pick a reference point, such as the lower-left corner of the target grid, and write down the horizontal (x) and vertical (y) position of each shot.
2. Average all the x values and all the y values. That point is the group center, sometimes called the centroid or mean point of impact.
3. For each shot, calculate its straight-line distance from the center: the square root of (x difference squared plus y difference squared).
4. Average those distances. That is the mean radius.

It is tedious by hand and quick in a spreadsheet. Several phone apps also let you tap each hole on a photo of the target and calculate it for you; whichever you use, check one target by hand once so you trust the tool.

A bonus you get for free

The group center from step two tells you where the rifle is actually hitting relative to your aim point. That is the number you want when you adjust a zero, and it is more reliable than "the middle of the group, roughly."

Vertical and horizontal: splitting the problem

At 100 yards a round group is a round group. At distance, the shape tells a story. Vertical dispersion grows with velocity variation and with inconsistent shooter position, while horizontal dispersion grows with wind and with how well it was read. That is why long-range shooters often care more about the vertical spread of a group than its overall size: it is the part the ammunition and the rifle control most directly.

When you log a group, write down the vertical and horizontal spreads along with the overall figure. Over a season, that record shows whether a problem belongs to the ammunition, the shooter or the wind. Our wind reading guide covers the horizontal side.

Converting to MOA and mils

Inches mean nothing without distance. Convert so groups shot at different ranges can be compared.

Example arithmetic: a 1.5-inch extreme spread at 200 yards is 1.5 ÷ (1.047 × 2), or about 0.72 MOA. The same group in mils is 1.5 ÷ (3.6 × 2), or about 0.21 mil. If you want the background on the two units, see MOA vs MIL.

Common measurement mistakes

• Quoting the best group. A single best group says more about luck than capability. Report the average of several groups, or better, the mean radius of all the shots combined.
• Calling flyers. Removing a shot because it "was me" is sometimes honest and often wishful. If you call a shot, note it before you look through the scope, not after.
• Mixing measures. Comparing your mean radius with someone else's extreme spread makes your rifle look better than it is.
• Mixing shot counts. A three-shot group and a ten-shot group are different tests. Pick one shot count and stick to it.
• Forgetting the bullet diameter. Outside-to-outside without subtracting a diameter overstates every group by the same amount, which hurts small groups most.
• Ignoring the cold-bore shot. If the first shot from a clean or cold barrel lands somewhere else, log it separately. Our cold-bore guide explains why.

What to do with the numbers

Measurement is only useful if it changes a decision. In practice, good group measurement helps with three choices: whether one factory load is genuinely better than another in your rifle, whether a rifle is changing over time, and whether a practice session improved your shooting. For the load question, compare loads with the same shot count on the same day, and treat small differences as noise until repeated results agree. Our ladder and group method article covers a structured way to run that comparison.

If you are testing factory match ammunition, a well-known reference load gives you a baseline to compare others against. The Federal Gold Medal Match 175-grain article explains why many shooters use it that way.

You can also browse the rifle ammunition category if you need candidates in another cartridge. Whatever you test, keep the targets.

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