Caliper
Also: Calipers
The everyday precision measuring tool of the reloading bench, reading lengths and diameters to a thousandth of an inch.
A dial or digital caliper measures outside dimensions, inside dimensions, and depth, typically resolving to 0.001 inch. At the reloading bench it is the tool you reach for constantly: checking cartridge overall length, confirming trimmed case length, and measuring bullet and neck diameters.
For the tightest work, such as reading bullet diameter to a tenth of a thousandth, a micrometer is more precise and repeatable. The caliper wins on speed and versatility, which is why it stays within arm’s reach of the press.