Ah...I had this problem with my 22-250....I decided to exchange the neck sizing bushing with the one from my .243 die and bingo....it solved the problem and gave me much better accuracy to boot. Most die makers will provide you with a slightly oversize sizing button at modest cost.
I think he's saying the shoulder is blown out too wide for the case body to fit into the seater?
Whose dies are you using and in what chambering? Can you take a caliper and measure your fired case diameter at the shoulder? The reason I ask is to see if one of us has a chamber drawing for your cartridge to see if your chamber is exceeding SAAMI maximum dimensions? If not, then the die is not made correctly, and I expect that if you sent the die an a fired case to the manufacturer they would likely take care of the problem for you. They may do that even if your chamber is off.
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