My buddy and I are going to play with my new H&R synthetic in .243, by having a new barrel fitted to his .17HMR, and then rechambering it for .17 Fireball, to use on my receiver. I had thought about starting with a .17HMR sportster, but would rather tweak the barrel fit than re-machine the firing pin block to make the rimfire receiver a centerfire. Reprofiling the ejector, or even machining a new one isn't a problem (We're both Journeyman Toolmakers, and I have all the shop machinery needed).
The reason I'm going for the Fireball instead of the .17 Remington is simple economics. I may lose around 300 FPS, but I only need 15gr of powder to get 4000 FPS. I'll get over it. I'm sure the critters I shoot with it won't be able to tell I'm using the slower round...
According to the specs we've seen, the Mach IV is ever so slightly longer in the neck than the Fireball, but otherwise the same, so Fireball ammo should, in theory, be safe in a Mach IV chamber, but not the reverse.
BTW, I recently saw an ad in Shotgun News, I think, or maybe it was American Rifleman, for Mach IV ammo really cheap...