Depends on several factors:

How hot are your loads? Have the necks been annealed? How badly oversize do they get from being fired in that rifle?

For starters, assuming the brass was originally fired in that samerifle, I would see if the fired cases would go back in and allow the bolt to fully close before doing any sizing. If they do, then all that's needed is sufficinet sizing to hold the new bullet. If they don't, then I would screw the sizing die in a small amount, size a case, test it forchambering, etc., etc.,until you reach the point where the caseWILL re-enter the chamber fully. You might find that this spot is prior to the point where the case is being entirely resized.

I had an M1 once in which I used a match load of the 168-grain Sierra MatchKing bullet and 47.5 grains of IMR 4064. My once-fired cases would go back in FULLY, WITHOUT ANY sizing at all. So I just resized1/3 of the necks, so theywould hold the new bullets...... cases lasted a long time!

IF you are getting incipient casehead separations in a beltless, rimless bottleneck cases after a mere five reloads, either your loads are WAY too hot, or there is something wrong with your rifle/cartridge combination! Like, the riflehas excessive headspace, or you are creating it with your FL die.