The target you just posted is certainly encouraging.
I have to ask. You said the brass was .010" too long. Compared to SAMMI length? Or .010" longer than your chamber end? I would suggest to you that you are far from the end of the chamber UNLESS you actually measured it. Most chambers, unless made with a custom reamer, have a huge distance to the end of the chamber compared to SAMMI max length specifications.
I would suggest there was something else at play here. Sure if all cases are the same length and diameter then they should have very similar bullet tension/ release characteristics which would help create consistent velocity and better accuracy. Might not be able to see this with a hunting sporter but everything helps.
What Varmint Al says on his site is true, I found the same sort of dimensions:
Quote:
"I usually find that the factory recommendations for case length leave a 0.050 inch to 0.070 inch gap between the end of the case and the actual length of the chamber."
link to how to measure chamber length.
Varmint Al's Handloading/Reloading Page
The question remains, what has the rifle shooting better now? I think you should repeat the testing for a larger sample.
This is the beauty of this site, Len has the notification system so we never miss an entry. We posters offer suggestions to the OP and we all learn. Keep us informed Creedmoor shooter.
Prime reason for a chamber casting.