Our group measuring attachments install quickly to your digital or dial caliper blades and enables you to measure your groups at home or at the range. Simply bracket the furthest two bullet holes from outside edge to quickly measure group size. Each Group Measuring device will measure three different calibers of bullets. The first model will measure groups of 22 caliber, 6mm and 30 caliber bullets. This device works great with our economy MIC-14 digital calipers.
Sinclair, Issue:5B, Page:237