Hornady Bullet Comparator and Complete Set With Inserts for Precision Measurement Used in conjunction with the O.A.L. Gauge, the bullet comparator provides the ultimate in precision measurement. Bullet comparator will properly align the O.A.L. gauge for proper precise measurement using your caliper. Comparator body and all 14 bullet inserts for bullet diameters: .172, .204, .243 (6mm), .257, .264, .277, .284 (7mm), .308, .338, .358, .375, .416 and .458.