That's a GREAT group vertical!
Sounds like you have the fired brass bump just right. You can verify by adding a layer of scotch tape to the base. See if your bolt flops. That's 0.002" per layer of tape, so ideally only 1 layer should stop the flop..
Best way to find the lands is to keep...
Factory rifle (ie, SAAMI)? or custom rifle (with potentially tighter tolerances)? Did the bolt handle drop with the fired brass before you bumped the shoulders? And the fired brass was in your rifle, right?
As a science guy I like the idea of applying statistical tools to all of this. Do you have the statistics expertise to take it further? (I don't but would understand the result!) Statistically how many ACTUAL shooting ladders or tests with how many rounds at each charge would it take to validate...
I have not read the entire thread, but what other loads did you do below but close to this maximum? Also, a suggestion. Don't load so many when you're trying to figure out what is ideal. You're going to waste a lot of powder, bullets, and even barrel life if the Max Load turns out to not be your...
Skipping ahead, so apologize if already covered.
The picture of the 400 rock seems to have only 2 hits. So, maybe it's not really "MOA" at 400?
Second thought, aim small-miss small. Paint a black 6" or less bullseye in the center of the big orange.