Years ago I was in McBride's, in Austin TX. I walked in to where they had all the firearms, and struck up a conversation with the salesman. When he found out I was a handloader, he handed me what was then a true wildcat: a 22 necked down to 17. (I honestly don't remember if it was a magnum or not, but I think it was probably a LR.) He told me there were several guys doing it: they had a piece of 1" Lexan bolted to their presses.
They were pulling the 22 projectiles, dumping the powder, necking the rimfire cases down to 17, then replacing the powder and seating a .172" projectile. The Lexan was in case the priming mix decided to go off during the seating stroke.
Seeing as I just bought a 5MM Rimfire Magnum, I really wish there were a place to get primed 22 Magnum cases. The 5MM case is 1.2" long so it is neither LR or Magnum length, but if the cases were available it would be easy enough to get someone to cut me a die, and I could trim the cases to length. The rest is simple, as 20-caliber projectiles are out there. Using the powder in a 22 LR as a guide, just work up...