We have been making these hex style bullet comparators for over 20 years, and believe we make the most accurate tools available. The holes in the Sinclair Bullet Comparators are cut with throating reamers to accurately duplicate the actual throat of a rifle. The comparator is used with calipers, and allows the reloader to accurately measure the length from the major diameter of the bullet to the case head. It also enables you to make accurate seating depth changes to your bullet seater. The Sinclair Comparator does not attach to your calipers, so your calipers are free for other measurements. Machined from stainless steel. There are two different models of Sinclair Comparators to cov Mfg: Sinclair International