Paul,
If you are planning to neck-size only, but in a FL die, the competition shell holders may not be tall enough for that purpose. In a chambering with a lot of taper in the case, like the 30-40 Krag, it may work out. In a case with little taper, like an Ackley Improved design, it almost certainly won't (though I doubt you could neck size an A.I. with an FL die at any setting). The competition shell holders only span a hundredth of an inch because that covers most head space tolerance. The problem you get into with FL dies for neck sizing is you have to limit how far you insert the case so it doesn't start touching the sides of the die. If it starts squeezing the sides, you no longer are sized for the chamber. With a large case taper, that's easy because you have to get pretty close to FL sizing before the sides of the taper touch. With almost no taper (i.e., the A.I. shapes) you may be touching the sides before the neck even gets to the neck portion of the die.
I saw a post reference an English shooting magazine that had done a review of all available neck sizing dies, and the reviewer's experience echos my own. It found all the conventional neck dies produced some lateral forces on the case that tend to pull necks off axis, producing neck runout. That's not an uncommon complaint, especially where an expander is used. Where an expander is not used the "dreaded donut" tends to build up sooner. The one exception they found is the inexpensive Lee Collet Die. It produced no lateral force on the cases, resulting in far less neck runout than any of the others gave. As a bonus, the mandrel it sizes the neck down onto prevents donuts from ever getting started.
I know it's counter-intuitive that something costing less than the competition shell holder set would actually do a better job than neck dies costing more than that same set, but it seems the fundamental design concept is just inherently better than the traditional approach.
The new Redding sliding-sleeve bushing neck sizing die probably was not reviewed in that article, and it may succeed in eliminating the lateral force problem? I haven't used or seen a review of one yet. But you can buy several collet dies for the cost of one of those. And it will still allow donuts to form.
The Lee Collet Die does take a little getting used to, but it isn't a lot. You may have to anneal necks more frequently to get the best neck tension from it (every three loadings seems most popular). You have to run each case into it twice (once, rotate 180°, then a second time), so it isn't suitable for progressive loading. But those are the only potential drawbacks I can think of. Not enough to stop me getting the straightest cases I can.
