Center to center is what we are talking about. However, since the centers are blown out of a bullet hole, you get the same linear separation of two bullet holes by measuring left-edge-to left-edge, top-edge-to-top-edge, etc. When you can't see the edges of both holes (for example two bullets through what appears to be the same hole) measure the greatest diameter and subtract the bullet diameter. For example, if the fat hole measures .375 inch in diameter at it widest point and the bullet is a .308 inch diameter (like a 30-06), you have .375-.308 for a .067 inch group.