From the OP's photos in post 51 it appears that the cartridges are not fitting flush on the lower step in the gauge, but protruding a little. I think there has been only one comment on that.
I'm thinking cfushing the shoulder and increasing the OD at the shoulder will interfere with the cartridge resting properly in the bushing die.
Once again folks,the OP said he tried his sized brass in the chamber and the bolt closes easily.
The sizing operation is successful.Put the sizing die away and forget about it.
Now,from sized brass that fits the rifle,the brass getspowder and then runs through the seater die.
So what changed during seating tho make the cartridge not chamber?
As Mehaveysuggested,it could be long seated,but the OP already tried seating deeper.Its asecant ogive bullet.
AndIMO,if it was a "hitting the lands" issuewe'd see uniform landshaped impressions.
It is possible,but I doubt it.
High primers? Possible.
But if its theseating op that makes thedifference,no part of theseater dieshould alter the cartridge in any way but the crimp.
If you think t happens another way,please explain the mechanics and the sequence
A whole lot of snipe hunting going on