I've used bushing sizing for necks longer than I can remember..
Your standard Redding/Wilson type bushing dies partially size necks, and the length of that much is adjustable.
As far as shoulder bumping/body sizing, this is additional with most 'FL bushing' dies,, and separate adjustable.
There are only a couple die makers offering dies with bushings that include FL neck and shoulder in them.
Some of us bushing or collet neck size with one die, and body size/shoulder bump with a body die in separate operations.
None of this is actual FL sizing, and that's fine.
For consistent neck tension, you should size no more neck length than seated bullet bearing communicates with. From here, you can reduce neck sizing length incrementally during load development, if desired. This length greater/smaller is the true tension adjustment (not seating interference amount).
All new/unturned necks have thicker brass nearest neck-shoulder junction. This is referred to as donut area, which changes over reload cycles. It's detrimental to seat bullet bearing into it, and just as bad to bring it into tension otherwise. Sizing of it does just that, regardless of seating depth.
Even with turned necks, bringing near shoulder neck area into tension is bad. If bullets seat through neck-shoulder junction, you've brought the shoulder angle into tension. If bullets are seated short, as normal, sizing length beyond this will only leave un-upsized neck area binding the bearing-base junction. Either way, this portion of necks will not expand the same as the rest.
It is not as consistent as the simple spring back gripping of bullet bearing, where partial length sizing.
There is actually nothing 'good' with FL sizing of necks, especially when it goes beyond seated bearing length.
While it greatly increases neck tension (to an extreme), just as well it increases tension variance (to an extreme).
There are only a couple underbores, small BR cartridges, that benefit with this anyway. For the rest of us, it's bad in every way. You should never do it.
