I am increasing neck run-out mostly from bullet seating. Run-out is consistently around .002" from sizing and then jumps up another .003-4" from seating Barnes tsx using RCBS dies. In your experience, what is the most likely cause? I have always had more run-out created in the sizing process but not in this case. Thank you.
I first experienced this 40 years ago.
The first thing you need to take a serious look at, is the press itself.
Change dies and cartridges and see if you are getting the same thing across all of your cartridges as some cartridges are less effected by this misalignment and others are not scrutinized as much if not perceived "Highly accurate" cartridges.
Also, take into consideration the age of the press you are using.........An older press is more likely to have misalignment between the die holder tapping and the shell holder, which can also mean the primer shaft alignment.
Naturally, it is/was more prevalent with turret style presses because of the multi drilling necessary to accommodate all of the dies. Having the turret head as an additional press component to align didn't help either.
John
