A 221 fireball will not work in a 527 hornet action.Good luck coming up with mag box, spring and follower for the .221. I have kept an eye on Rems' rifle parts web site for a very long time waiting for them to manufacture some more. The .221 probably won't work in a CZ 527 that was made for the Hornet, as the Hornet is a rimmed cartridge and the .221 isn't. Just to be clear, the .223 isn't a "boomer" by any stretch of the imagination,,,,,, the .222 isn't, either..............
Different boltface and magazines, I believe the extractors are different also.