In precision target shooting
matches, the group size is the distance between the two farthest hits on the target minus the caliber
of the bullet used.
For example, two shots that make two perfect holes each touching each other at a tangent would have a group size of 0 (inches or millimeters, depending on what you use to measure.
Standard group matches use three shot groups at different ranges. The person who can make the smallest group wins. Three shots that more or less go through the same hole produces a pattern that lookes like a cloverleaf and is called such. Group matches is not only about accuracy but consistency as well.