I wanted to go over another matter about neck expansion. The 'why do it' part.
Up front, I'm not a metallurgist, so I can only generally describe this.
When we size brass(in any way, anywhere) we're adding energy. The brass is no longer in it's lowest energy state and it wants to get back there. Spring back is an immediate evidence of this, but what many don't consider is that spring back energy does not reach a balance all at once. There is more of it in play, over a longer period of time (way slower rate).
Brass continues to move (where it can) opposite of last energy added. If it is not allowed to move, it will just always want to, holding a bias of energy. So, when we decide to skip expansion as a last action, we're leaving necks sprung back outwards, and biased to continue creeping outward. If your sizing provided the interference you wanted right off the bat, it won't be like that a month from now. And your shoulder bumps will change, and so will your primer seating.
Consider why we have always expanded necks, as a standard.
When you make ammo today, for later use down the road, you should expand necks (what I call pre-seating).
With this bias set, necks will tighten down the road, or at least grip bullets with the same force, instead of a lowering force. I'm sure ammo makers have been aware of this for longer than most of us have been alive.
I mentioned earlier that pre-seating expansion would ideally take necks to cal, and released to spring back into ~1/2thou interference. This is not only ready for bullet seating, but an incredibly stable condition.
When you seat bullets in these necks, they will expand necks only against spring back, and there will be no up-sizing (no yielding). The inward spring back bias is locked, unchanged, right there. Perfect.
You could rely on this today, and weeks, months, years down the road.
A really good process anneal (stress relieving) is effective to remove much of this character in brass. But of course this changes what you get from brass (it's character), so it is only a good thing while it tests as such.
Annealing is not a fix-all.
While annealing increases seating force, it lowers neck tension forces. So for example, if your load likes high neck tension, then it can punish you for an annealed condition.
You want stability, and to load develop with it, and be able to manage it.
I've found that it's easier to manage through minimal sizing, but a good annealing plan may work as well for you.
With a cartridge that has to be sized a lot, like a 30-06 for example, then I would plan up front for frequent dip annealing.
