Unless you REALLY want to go to the trouble of annealing, forming, sizind, and trimming inside and length...just buy the brass. It is not expensive or hard to find and you'll spend more in tooling to convert the .223 over then you'll save in brass cost even if you don't include your time as a factor.
For forming thought, the easy way is to anneal the .223, use a bull berry double ended forming die to neck it to .17, trim to length, OD (or ID) trim the neck to proper thickness, and then FL size. And the lower half on the neck that you pushed back.