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A good full length sizing die can produce concentric cases but some do not. The issue is almost always the expander ball. If you have a die that is making your case runout more than .002 - .003". Try taking guts out and sizing. If runout is minimized you have your smoking gun. Of course your neck is now too tight and will need to be expanded. A lee collet die can be used for this. I like working my brass as little as possible so boring out the neck of the fl sizing die in a lathe or getting a body die is the answer. This all works along with no case cleaning well. If you are having success with your methodology that is great. I am just providing some different perspective after many rabbit hole dives.