caliber-rule

from The Century Dictionary.

• noun Gunners' calipers, an instrument in which a right line is so divided that, the first part being equal to the diameter of an iron or leaden ball of 1 pound weight, the other parts are to the first as the diameters of balls of 2, 3, 4, etc., pounds are to the diameter of a ball of 1 pound. It is used by engineers to determine a ball's weight from its diameter or caliber, and vice versa.
• noun An outside caliper formed by a rule having a graduated slide with a projecting foot, between which and the end of the rule is placed the piece to be measured. Also caliper-rule.