Forming .221 Remington Fireball from .223 brass is feasible. I have a Redding die set that does that. Once the case is formed, you will have to thin the case neck. The new neck is being formed from a part of the brass that was formerly a part of the body of the .223 brass, and it will be too thick for the neck. An outside neck reamer that attaches to the Forster case trimmer will do that easily.
That same Redding case form die set can be used to form .222 Remington brass, in case yoiu need some of that. Either use a spacer 0.190" thick, or simply re-adjust the die lock ring for the .222.
I got into the case forming when I was forming cases for the .17-222, the .17-223 and the .17 Mach IV. Since I had a lot of case forming, I decided to start with the cheapest brass around, once-fired military .223 brass. That worked quite well.
If you get started forming brass from the .223 and your case losses from case splits are too high, you might anneal the brass before you start these extensive forming operations.
Jim.