Center to center.
That is the true differencein the actual points of impact.
This way all calibers are kept on an even playing field, and the shooters skill, and quality of matching of the gun to the load are more acurately represented.

You can say "I can cover my group with a quarter, or a tennis ball", or whatever.
That's different.

I feel thatgroup size data is only of value if you record the distance between where you made the bullets hit, not how much paper theytore upwhen they got there.

That's why competitors use the largest diameter bullets they can shoot well or that are allowed. They will cut scoring lines better when the shots are not hitting where they should.