Did you try a shim under the case?
This will tell you if it's the die not sizing your brass ENOUGH.
I have been loading for belted magnums for a LONG TIME, it is not uncommon to find a short chamber and a long die in shoulder length. Because headspace is controlled by the belt, the head to shoukder length is NOT held to tight tolerances, therefore, as you size a fired case it lengthens making it too long for your chamber if your chamber head to shoulder length is short.
Another question, are you camming over on your press and is there a gap between the shell holder and die bottom when a case IS BEING SIZED IN THE DIE?
Adjusting the die to touch the shell holder at the top of the stroke is incorrect, when a case is being sized the shell holder will NOT be touching the die due to slop in the linkages and true FL sizing will not be happening.
I would be checking your die adjustment before I wasted money buying another die set.
Cheers.
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