Bullet seating measurement...CBTO or shoulder datum point to ogive?

[D-4297]

As long as the prepped cases have a very consistent cartridge base-to-shoulder (CBTS) measurement and you consistently bump the shoulder back .001" to .002", it really doesn't matter. In my hands, CBTS is typically one of the most consistent measurements I take during the reloading process. One complaint regarding the Hornady OAL gauge is that the brass isn't necessarily dimensionally identical to fired brass from a given chamber. Again, it doesn't really matter. If you take a CBTO measurement with the bullet just barely touching the lands using the OAL gauge (or any other method), you will use that as your reference/starting point in order to set up a seating depth test.

From that point on, you can actually measure the CBTO of all loaded rounds with very good accuracy, to at least +/- .001". Because you will actually be shooting those loaded round in a seating depth test, the target itself will tell you what the optimal CBTO or seating depth measurement is. At that point, issues such as slight differences between the brass case in the OAL gauge and brass fired in a given rifle effectively disappear. The initial measurement is only a reference value, nothing more.

As Ned has said ; It's a reference , or starting point . It allows testing for load value , and then allows for adjusting the seating depth from that referential starting point . I found that the closer I have my cases Bumped , to minimize headspace variations , the easier it is to bring everything else in line . Since I anneal after every firing , Bumping .001 isn't a crisis . Good case prep makes for consistent ammo .