So I converted a 204 to 17 fireball. The difference is the button metal on the 204/223 guns has the slot for the metal magazines about 1/8 inch longer. In other works if you have a 221 rifle the longer 223 magazines will not fit.
The challenge for me was actually finding the correct bottom metal so i could run 221 magazines. I got lucky and found someone with an extra. I put that on and have been looking eBay over the last year and found mags.
With that said if you wanted to ever run the longer mags I do have an extra 223 bottom metal. They will kinda function with 221. I recommend a quick fast cycle of the bolt and adjusting the front and back mag spring tension by stretching or compressing. Not near as good as just running 221 mags.
If you found a 222 mag it might work. I would be interested to know
