Re: Headspacing Comparators
Yes, they will fit. You will not,however, be able to measure the shoulders on a loaded cartridge with the Sinclair insert installed. The Sinclair insert is just enough shorter than the Hornady insert to prevent measuring loaded rounds. The bullet tip will hit the caliper jaws.
Sinclair does make their own comparator body in a extra long size to measure loaded cartridges with their shorter inserts.