How to measure shot group size: extreme spread vs mean radius
One group can produce several honest numbers. Learn what each measure includes, how to calculate it and when comparisons become misleading.
Published · Updated · Written and source-reviewed by the TargetIQ editorial team
Start by naming the measurement
The phrase group size is incomplete unless the method is stated. Extreme spread, outside-to-outside span, mean radius and horizontal or vertical spread can all describe the same holes with different numbers. Record the method, unit, shot count and whether a called flyer was included.
For 4.5 mm airgun pellets, a clean outside-to-outside caliper reading of 12.8 mm becomes a 8.3 mm centre-to-centre extreme spread after subtracting 4.5 mm. Do not subtract the pellet twice: both outer radii together equal one diameter.
| Measure | Calculation | Best use | Main weakness |
|---|---|---|---|
| Extreme spread | Largest distance between two shot centres | Familiar group-size comparison | One flyer controls it |
| Outside-to-outside | Caliper span across farthest hole edges | Fast physical measurement | Includes projectile diameter |
| Mean radius | Average distance from the group centre | Uses every shot | Harder to calculate by hand |
| Centre offset | Average X and Y position | Sight and placement diagnosis | Does not describe spread |
Calculate extreme spread from coordinates
Treat the target centre as X = 0 and Y = 0, then record each shot centre in millimetres. For every pair of shots, calculate the straight-line distance using the difference in X and Y. The largest pairwise distance is extreme spread.
A three-shot example at (0, 0), (3, 4) and (-2, 0) has pair distances of 5.0 mm, 2.0 mm and about 6.4 mm. Its extreme spread is therefore about 6.4 mm, not the width or height alone.
Calculate mean radius and group centre
Average all X coordinates and all Y coordinates to find the group centre. Then calculate each shot's distance from that average point and take the average of those distances. Because every shot contributes, mean radius describes the group more completely than the farthest pair alone.
Mean radius is not the average distance from the target centre. Moving the entire group left changes centre offset but leaves its internal mean radius unchanged. That separation is precisely why the two measurements work well together.
Make comparisons that mean something
- Use the same shot count; a 60-shot group has more opportunities for a distant shot than a five-shot group.
- Use the same event and units; Air Rifle and Air Pistol target scales are not interchangeable.
- Include all shots unless an exclusion rule was decided before seeing the result.
- Record conditions and equipment changes that could explain a difference.
- Look at several sessions instead of declaring improvement from one unusually tight group.
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