Home Forums Reloading Big Game Rifles C.B.T.O and measuring to Datum Line Question

Hey y'all kind of stumped on something, and could use some advice.

I'm somewhat new to reloading and just starting to actually develop a load ( I have read the sticky as well as watched a lot of youtube videos). Loading for 6.5 Creedmoor.

I've measured C.B.T.O of my rifle using the hornady comparator and modified case, however i am using new Norma brass to load.

Hornady Modified Case Measured to Datum Line = 1.5535"

Norma Brass ( Brand New and Not Sized) measured to datum line = 1.557"


So when loading the Norma Brass should i add or subtract the difference between the two above measurements to the C.B.T.O that was taken with the modified case?

Ignore the shoulder datum line measurement (until you go to set up your sizing die). Just go with the OAL to the ogive for bullet seating.

Thanks.

So I guess measuring to the datum line is something only really necessary when dealing with once fired brass and determining how much the case stretched and how far back you need to bump the shoulder?

Yes, I think you've got the idea. I try to bump shoulders back only about 0.002" or so to attain a nice fit to the chamber and to extend case life whether I'm using a FL die, a body die, or whatever. If I think the bolt is closing a little too hard then I adjust accordingly.