What steps or procedures do you envision using these mandrels for, specifically?
I ask this because you might want different sizes for different steps in terms of brass preparation.
For example, I also use the 21st Century mandrels and would suggest these three as a set that can be used to cover a few different applications:
1) as a final neck-sizing step to generate .002" neck tension - 0.3065"
2) neck turning - 0.3060"
3) removing "flat spots" in necks (such as can be caused by a strong ejector spring) - 0.3100" - 0.3110"
If you're solely looking to size necks at different neck tension (interference fit), then a mandrel that is .0015" under bullet diameter will yield approximately .002" neck tension, .002" under bullet diameter will yield approximately .0025" neck tension, .0025" under bullet diameter will yield approximately .003" neck tension, and so forth. You will generally obtain about .0005" more neck tension than the amount the mandrel's diameter is below bullet diameter due to spring back (contraction) of the brass as the mandrel is withdrawn. So you could easily select a few different mandrel diameters based on the range of neck tension you might wish to try. I generally use .002" neck tension and nothing else, and so the 0.3065" mandrel pretty much covers that for me.
Just remember that you want to use a bushing die first with a slightly smaller (~ .001" to .002" smaller) bushing than you might choose if it were to be the sole neck-sizing step. That way, the mandrel will actually do some work when opening up the necks. If the mandrel is too close in diameter to the neck ID after the bushing die step, it does not seem to do as consistent a job in setting neck tension. If you were solely interested in using mandrels to set neck tension, I might suggest buying a set of something like 0.3070" (~.0015" neck tension), 0.3065" ( ~.002" neck tension), 0.3060" (~.0025" neck tension), and 0.3055" (~.003" neck tension). If this is your goal, just remember that you may also wish to purchase some additional smaller bushings, if you don't already have them. As I mentioned above, you likely want to use a slightly smaller bushing when the final neck tension will be set using a mandrel. However, you don't want or need to grossly over-work the necks by using too small a bushing, so having a few different sizes might be a good idea if the primary purpose for the mandrels will be to generate various neck tension settings. The other thing to remember is that you don't have to purchase a "complete" set all at once, but can add a mandrel here and there as some new use crops up. The 21st Century mandrels aren't so expensive that it is too painfull to add a new one here and there.
