You want the turning arbor, not the expander mandrel. It’s a little confusing, but the turning arbor gives 0.002” neck tension, so that’s what most folks use who go the mandrel route. The Expander Die Body is what you mount the mandrel in, and it’s nice because it’s almost universal, you just swap the mandrel inside to go to a different caliber. The have an Extended one for really tall cases.
Other folks are right that there are options to make it a 2-step process; some dies will let you remove the expander ball from the decapping spindle (Forster maybe?), so you FL size and decap in one step, then mandrel in the next. This is definitely a time saver, but I’ll also say that I’ve occasionally appreciated the ability to decap and nothing else, like when I’m taking post-firing headspace measurements to set shoulder bump. Sometimes the primer can sit a bit proud and throw off this measurement, although I’ll say I’ve not seen that.
As
@spife7980 noted, there are also bushing-style sizing dies that only squeeze the neck down a specific amount, leaving it at the desired tension. Forster (maybe others) offers a honing service on their FL sizing dies that accomplishes the same. These options do have the drawback of being brass-specific, since your tension is set by the bushing size and the neck wall thickness, which varies between mfg’s. It also pushes any irregularities into the bullet bearing surface, rather than leaving them on the outside of the neck where they have theoretically less impact on accuracy; however, I’m not convinced that this concern actually has much of an effect, but it’s not like I’ve tested it.
One last comment here, some have talked about cleaning after sizing and before seating bullets; there’s a thread on here discussing bullets cold-welding to the case neck if left alone too long without lube in the neck, so if you’re gonna leave ammo sitting a long time, you might check out that thread.
