Seating depth with new and once fired brass

[CanMike]

I think I get the math but wanted to make sure before I seat some bullets.

I found seating depth on virgin lapua brass as best at 0.010 off lands. Let’s say CBTO of 2.200

Once fired brass shoulder to base grows 0.006

So in order to get same seating depth I need to load the once fired brass 0.006 longer CBTO to 2.206

Am I understanding it correctly?

Mike

 

[spife7980]

Am I understanding it correctly?

No, well maybe. Depends on how you arrived at your initial measurement. The bolt face to the point that the bullet touches the lands hasnt changed.
If your measurement was to the case head instead of the bolt face such as with the hornady tool, then yes, adjust accordingly.
If you do the wheeler method that pushes the bras up against the bolt face, then no, dont change.

 

[RegionRat]

If there is a 0.006" difference between your fired brass and your virgin brass, the answer depends on several other variables and that means this is another "it depends" answer....

You might shoot that virgin recipe and tell us you can detect some "significant" improvement based on your 2.200" recipe.
The question would be, how wide was the window to begin with when using virgin brass? Was it sensitive to less than 0.006"?

Let's say you are now shooting fired brass and you minimally bump it. Will the change in the brass cause the difference or will the shoulder distance to the lands cause the difference?

You can tell where this is headed, which is to say that only a firing test will tell since you are also making changes to the brass.

Many guns and shooters cannot detect the difference if they leave the CBTO value the same when you consider the difference between their virgin and fired brass, while others can demonstrate it. YMMV

 

[CanMike]

Thanks for the answers.

How wide the node is I don’t know as only did preliminary seating with the virgin brass. Finalizing with fire formed.

10 was better then 15 off which was better then 20 and 25 So I am guessing my window is 5-12 or so as groups started tightening at 15 versus bigger jump.

I measure lands with both Hornady tool and case with bullet / close bolt. Both showed similar numbers. The Hornady modified case is almost identical dimensions to the virgin lapua brass.

Mike

 

[CanMike]

Also should add I am not set on a specific jump as that’s kinda arbitrary and not 100% accurate with measurement tools I have. I just want to match seating depth between the virgin and one fired. Basically want to replicate distance with the once fired brass as I have with virgin.

Mike

 

[Sig Marine]

Bullet distance to the lands is one thing. Case volume is another.

 

[CanMike]

Yup. I’m going to be adjusting powder charge a few tenths. Thats why I wanted to keep jump same and the. Adjust powder to get back to same velocity. Figure probably .2-.3 less powder. But will play with that.

Mike

 

[Sig Marine]

CBTO measurement changes pressure as does the powder charge. I look at powder as a “course” tune and seating depth as a “fine” tune. Finding each window offers some flexibility with a particular load for a specific barrel and bullet combination.