you'll need both FL and neck size dies. I am not certain they should be crimped, but opinions vary. If you FL size, the cases will go back to SAAMI chamber specs and fit any rifle, if you neck size the body of the case will fit the chamber exactly. The problem is that if you use the cases a couple times they'll get to fit so well, you won't be able to close the bolt on them. At this point you'll have to FL size them again and then be able to neck size for a couple loadings.
YOu'll definitely want to check the unprimed cases in the rifle after any neck sizing to make sure that the bolt closes. I'd say do it with each case, so you don't get stuck in a situation where you need that next round and can't get the bolt closed.
Generally, you'll get a few firings in between FL sizings and will not have to trim after neck sizing or not as much. I am not sure how a RUM will behave, I used to neck size my 22-250 all the time, but the RUM is a pretty intense cartridge. I also neck size with the 264 win and 300 Win. I did have a generic neck sizer from hornady that worked for my 30-378 and it seemed to work well, but the cases had to be FL sized when I got a different rifle.
A word of caution: It is never a good idea to neck size if you are planning on hunting dangerous game with that ammo. It could cause a jam that would cost you your life. This is the same reason that the 375 H&H has such a long tapered case, to allow the smoothest possible feeding in situations where there is teeth or tusk on the other end.
JT.
