Is this a head space issue?

[coldboreshot]

I'm leaning in the direction on those who have spoken about the chamber creating the case markings. Can you report back after you sample the Lapua brass to see if it occurs again.
Curious though, how much is the shoulder set-back during FL sizing?

 

[Dr. Matt]

Those lines look like they could be coming from a rough cambering. Do you have access to a Teslong?

Delayed update. I got a borescope and yes those are 100% chamber marks.

 

[chop house]

Delayed update. I got a borescope and yes those are 100% chamber marks.

And thank you for the update

 

[coldboreshot]

No worries. If you bore scope the chamber I think you will see what caused those lines. I see no problems.

BTW, My 250ai chamber leaves far worse lines than that. Almost sent the barrel back but I was able to clean it up.

I was leaning in this direction as well. A little pressure may be causing chamber maching imprints on the brass.

 

[boogerruger]

Sure looks like reamer lines. I've got a 6ARC from uintah and their chambers are pretty tight. I usually can't get to the 2nd or 3rd highest load Data before I get pressure signs.