New acquisition shows fired cases to have head expansion, but more so on one side
this is because the chamber is sloppy, a bit oversized so when the case is loaded it lays on the bottom of the chamber and when fired blows out more where there is room, the fix is a rechamber with a tighter fit reamer, I have heard of people getting around this by using thin tape at the case head so the case head expands equally when fired so it forms to the chamber, after only neck sizing to keep the body shape, the fix is a theory I cant say is true or not
when I full length re-size, changes shoulder shape and angle significantly to closer to what the book drawings show.. The re sized cases are definitely longer, as they are slightly harder, but not out of the question harder, to close the action on, meaning the newly formed shoulder is probably making contact with the chamber throat.
I suspect you are trying to take out the lop sided bulge in the case head when fls'ing but if you back the die out a bit the case will fit easier because you will not be crushing the shoulder down that is bulging the shoulder slightly making the case harder to chamber
as No1_49er suggested neck sizing would probably be the go
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