Just "posting out loud here"
That short little neck has me concerned for necking down. I just can't visualize how I would be able to neck down to almost half the .30 cal neck and avoid wrinkles, non-concentric necks, and uniform necks.
That said, I form 7.62x25 brass from .223 brass. Trimming .223 "too long" (how long, I don't know) and then necking it down to .17 *might* work on annealed brass. After turning the long neck for thickness and concentricity, the brass could be trimmed to length.
Another thought, how about a 17-.223 wildcat? (Edit) already been done, 17 Caliber Wildcats