What am I doing wrong here!!

[Kmccord]

Look at the 21st Century Expander Mandrel, they are not expensive, get the Mandrel Die and try the .262-.263 mandrel. If you wanted, you could buy the whole set that is 261 - 263 and will have them in .0005 increments. I have noticed, using the .2625 mandrel, it will spring back to .2620 ID measurements with my calipers.

 

[JTB]

I wouldn't overthink this. Lapua brass needs to be prepped by running an expander button through the neck prior to seating (unless there is a visible case deformation no need to full-length resize; just run case into sizing die far enough to allow the expander to pass through the neck). You need to remove the brass that is galling to your expander button and make sure you are applying a graphite-type lube in the neck prior to expanding. Run it through a couple of times if necessary (with graphite applied). Reapply graphite inside the neck and seating should be no problem with great potential for first firing accuracy. I normally bed my seater stems with a quality epoxy to prevent marking/deforming the jacket/ogive. They may still be a little tight but should still seat smoothly.
After first firing I much prefer using FL bushing dies to help assure consistent seating tension is maintained.