Measuring cartridge lengths across the bullet tips can cause variations from one round to the next. A Bullet Comparator allows your caliper to measure off the bullet’s ogive to eliminate the variation. Also checks bullet uniformity. The Lock-N-Load Comparator attaches to the blade of your caliper with a thumbscrew and uses interchangeable "inserts" available for .17 to .45 caliber. Popular basic set includes comparator "Body" and 6 insert bushings in diameters: .224" .243" .257" .277" .284" .308".