Measuring cartridge lengths across bullet tips is not a reliable or repeatable method for measuring your loaded rounds. It's common for variations of up to .025 to exist from one round to the next. Our Bullet Comparator solves that problem by measuring rounds from a reliable surface on the bullet the ogive to provide consistent precise measurements of your rounds. The Bullet Comparator is also used to check uniformity of bullets from base to ogive. - The Bullet Comparator easily attaches to the blade of your caliper with a thumbscrew and uses interchangeable inserts to measure from the bullet ogive. - Comparator body and 6 bullet inserts for bullet diameters included: .224 (5.56mm) .243 (6mm) .257 .277 .284 (7mm) .308 (7.62&8mm).