Chunk shooting is always measured center of bullet to center of the X. As you can see from the target, you "declare" the caliber that you are shooting for the scoring crew. Obviously this levels the field when a .40 shoots against a .50. The scorers use overlays on paper targets; when people shoot boards with knife cut Xs you are supposed to bring a half bullet with the center marked to help measuring, especially on shots that cut the X. When I "shot centers" on boards in Missouri, they didn't bother measuring shots that didn't cut the center. In a match like that there is no string measurement, closest shot wins First, next closest wins second, so it is possiple (but not likely unless you are Alvin York) to win all of 1st through 6th place.