I am getting more than a few split necks when using rem brass. I decided to get a Redding form die to allow me to form from .204 Ruger or 223 brass. The Redding die seems to work, but when I try to size with a .17 Die, the brass collapses at the shoulder. I have lubed everything, but in every case, the brass is useless. I have trimmed them to slightly over SAMMI spec.

You need at least one or more intermediate form dies to take the neck down in steps. I can form 17 Ackley Bee from .218 Bee but it takes two dies.

Was your collapsed brass once fire .223?
You might be more successful with new brass or brass that has been annealed just on the neck.