It looks like a very loose chamber. Your brass is growing a lot, especially length and necks. Mine grow less than .003 in length each firing. I would change bushing size so your loaded necks are .295 to work the brass less. Aside from that, I don't see anything off unless maybe there is a bulge in your chamber. My base-shoulder measurement on fired brass is 1.626, but that is probably just different comparators.

Are you using imr 4350 instead of h4350? The varget load seems very low too but I never messed with it shooting heavies. I used 36.5g in a .220 swift to push 55 gr bullets.

maybe change powders? Rl17 or h4831sc are both pretty easy to get and great options in .260.