Center-to-Center
Also: CTC
Measuring group size as the distance between the centers of the two widest shots, the standard method for scoring a rifle's accuracy.
Center-to-center is the conventional way to measure a group: you find the two holes that sit farthest apart and measure from the center of one bullet hole to the center of the other. Because the measurement targets bullet centers rather than outside edges, it removes the bullet’s diameter from the number and reports the true spread of the shots, which lets you compare groups fired with different calibers on equal terms.
Many shooters take the easier outside-to-outside measurement first, then subtract one bullet diameter to arrive at the center-to-center figure. That distance is what gets converted into minute-of-angle to judge whether a load is sub-MOA, though it only reflects the two widest shots and tells you less about consistency than a mean radius calculation would.