I feel for you brother! It took me 25 or so loads before I found one that would both give me at least 2100 fps and 3" accuracy at 50 yard for my .475 Nitro. I finally went to the old IMR 3031 formula and poly filler and it worked just fine.

Still, I wonder at the sanity of someone who shoots a .375 all morning and then picks up a .577 for dessert. I thought Mbogo375 and I were the only guys crazy enough to do something like that!

My double .475 Nitro shot light loads of Re-15 into the same ragged hole, albeit at 1750 fps or so.. and didn't come back to center until I got loads up to the max.. and not with Re-15 at that (IMR 3031 was the fix). 4350, 4831, 7828, Re-19 and Re-22.. none worked.

My limited experience with "load" regulating two doubles seems to show that if you can get the gun to shoot close with reduced loads (as you have).... never fear... somewhere, there is a load that will be right with a max load... Just keep on plugging and buy another shoulder or something.

WARNING WARNING
I WENT BACK AND LOOKED AT THE "FINE" PRINT. It was not IMR 4831 but H 4831 lot number 3074. H 4831 burns slower than IMR. My post above has been edited to reflect this change.

Make sure your primers are fully seated, and you might try magnum primers with those big charges of powder. Check your firing pins and the springs too. If the primer doesn't get a good whack, it can can velocity disparities. Also check your bullet crimp, it should be the same on every round. That usually requires that your cases be trimmed to the same length.