Sometime's to salvage brass, ya really do need to neck size, ie excessive headspace. FL size can be another thing though but to fit the case to your chamber you do need to neck size a couple time's. As said earlier, the case will get to tight for the chamber and be hard to close if you can close it at all. At that point you set the FL die to fit your chamber. Back the die out a couple turns, size a case and try to chamber it Likely won't go. Turn the die down about a quarter turn at most and do it again. Keep doing it until the case chambers with no problem. You'll have set the whole case back just a bit to make it fit your chamber. Lot of guy's call it bumping the shoulder. Good enough but the whole case get's sized just like the shoulder. I watched a guy doing it on a video and he recommended removing the firing pin when you do it. I never have done that and don't see any reason at this point to do it. One thing about it though, you need to set that die right there, lock it in, and use it like that, on that rifle only. It will then always make the case fit the chamber. Like to call it partial sizing but in reality it is not partial sizing, it is FL sizing the case to fit that rifle. The shoulder will be bumped back just a bit and the die will connect withthe whole case, impossible for it not to.