Check which version you are getting...
Audrey has had the official NBRSA calipers made by Neil Jones in the past.
Some recent calipers I've seen (like the new version pictured in the Sinclair catalog) have aluminum sliding plates with half-circles cut for each caliber... tolerable for "b"enchrest, but not suitable for "B"enchrest in my opinion. A transparent plate with inscribed circles for the calibers makes for a more accurate device since you can center the circle on a single bullet hole and visualize if the bullet is making bigger or smaller holes than the caliber suggests, and from there know how much paper and smudge needs to be included with the bullet hole.
It is pretty easy to design measuring devices that give us smaller groups, but the point is to accurately measure the real "center"-to-"center" distance between the widest shoots, not just tally the smallest group your device can measure. Just because a gadget automatically subtracts the caliber size does not mean that it will necessarily help you line up on the exact center of the opposing bullet holes. My advice to new scorers is to measure every group as big as they possibly can... get them out of the habit of snipping off .015 of black smudge that is common when they measure the bugholes and a .115 shows up on the wall as a .099.
Rod