The Sinclair Bullet Comparator inserts are meant to be used with the Sinclair Bullet Comparator and Bump Gage Body. These inserts are stainless steel and machined with a taper and a short straight section to more accurately duplicate the chamber throat. This eliminates the wobble experienced with many comparators which have standard drilled holes. Our design allows you to quickly and accurately measure and compare bullet ogive-to-base as well as loaded round ogive-to-cartridge-base measurements. This measurement gives more consistent results than measuring overall lengths since the meplat (the open tip of the bullet) can very by several thousandths of an inch from bullet to bullet.
Big Book Catalog, Issue:68, Page:531
Sinclair, Issue:5B, Page:027