How can group size be smaller than calibre?

Q: I recently read where benchrest competitors often shoot five shot groups measuring .180 inch and smaller with rifles in 6mm PPC.

How is it possible for a group to measure less than the diameter of a .243” bullet?

– Paddy McFadden

A: Group size is generally measured from centre to centre of the two bullet holes that are farthest apart. Therefore, if all five bullet holes precisely entered the same hole in the target, the group would measure .000 inch.

This has never happened in a benchrest competition, as far as I know, but several shooters have come very close through the years.

There are two ways to determine the size of the group. One way is to measure from the inside of one hole to the outside of the other with a caliper.

If the group is too small for that method, measuring from outside to outside gives the same results if you subtract the calibre size from the measurement.

For example, if a group fired from a rifle shooting .243” bullets measures .423” from edge to edge, its size for the record would be the difference between the two, .180”.