Same thing with jacketed bullets, leaving a smaller hole than the bullet's diameter. This seems to happen, no matter the backer or no backer.
Thus, when measuring a group that is a single hole with the outside to outside measurement, then subtracting to get the centre to centre measurement, I usually
subtract the actual diameter my ball/bullet makes in that same paper, same backer, that same day - just to be accurate.
![Cool 8)](https://americanlongrifles.org/forum/Smileys/default/cool.gif)
This is sometimes as much as .010" under
the ball or bullet's actual size.