Everyone quotes group size, but it might be the noisiest way to measure how a rifle shoots. The mean radius vs group size debate matters because picking the right metric means fewer rounds wasted and fairer comparisons between loads. Here is how each works and when to use it.
In this guide
- How group size is measured
- The problem with extreme spread
- What is mean radius?
- Mean radius vs group size
- CEP and R50
- How many shots do you need?
- Which should you use?
How group size is measured
“Group size” almost always means extreme spread: the center-to-center distance between the two shots farthest apart, usually quoted in inches or MOA. It is quick to measure with calipers, which is exactly why it became the standard. If you need the how-to, see how to measure group size in MOA.
The problem with extreme spread
Extreme spread has two weaknesses. First, it is defined entirely by your two worst shots — one flyer doubles it, and it tells you nothing about how the other shots clustered. Second, it grows with sample size: a 10-shot group will almost always measure larger than a 5-shot group from the same rifle, simply because more shots give more chances to catch an extreme pair. That makes it statistically noisy and unfair to compare across different shot counts.
What is mean radius?
Mean radius is the average distance of every shot from the group’s center (the centroid of all impacts). Because it uses all of your data instead of just the two outliers, it is far more stable and repeatable from group to group, and it barely flinches at a single flyer. In statistical terms it is a more efficient estimator — you get a trustworthy read from fewer rounds, which is precisely what a handloader testing many charges wants.
Mean radius vs group size at a glance
| Metric | Uses | Strengths | Weaknesses |
|---|---|---|---|
| Extreme spread | 2 widest shots | Fast, caliper-friendly, familiar | Noisy; grows with shot count; ignores the cluster |
| Mean radius | Every shot | Stable, efficient, fair across sample sizes | Needs software to measure each impact |
| CEP / R50 | Every shot (probabilistic) | Robust, statistically meaningful | Less intuitive to most shooters |
What about CEP and R50?
You will also see CEP (circular error probable) or R50 — the radius of a circle, centered on the group, that contains 50% of your shots (with R90/R95 variants for more). It is a probabilistic cousin of mean radius and a very honest way to express precision. If you want the formal background, see circular error probable.
How many shots do you need?
Because mean radius uses every impact, it settles down with fewer rounds than extreme spread does — you can get a trustworthy read from a 5- or 10-shot group, where extreme spread would still be swinging from group to group. That efficiency is the practical reason to prefer it during load development: you reach the same confidence while burning less barrel and brass. Extreme spread, by contrast, keeps creeping upward the more you shoot, so judging a 5-shot group against a 10-shot group on extreme spread alone is comparing apples to oranges. If you only remember one thing: compare like sample sizes, and lean on mean radius when rounds are precious.
Which should you use?
For a quick caliper check or to compare against published group sizes, extreme spread is fine. For serious load comparison — deciding which charge or seating depth truly shoots better with the fewest rounds — mean radius (or CEP) wins. LoadNode reports both, in MOA and MIL, from a single photo of your target, so you never have to choose between the familiar number and the better one. Then cross-check the velocity side with a good SD.
Handloading is an adult activity. LoadNode is a logbook and analysis tool — it never provides load data. Always develop loads from current published data, start low, and work up safely.