Thanks Daryl_S
I really am measuring the Hornady #2640 @ .258" +/- .0005" certainly not the.263" you mention and which I expected to see.
I don't have any issues with cases not obturating correctly.

I'm 100% confident my reloads should have shown by now at least something looking half respectable, so, something else is amiss.
Whatever is wrong is not showing in anything to do with scope, mounts or any other obvious aspect of the rifle other than bullet or barrel or both.
Cheers, d