AFAIK the .221 Fireball was only made in the original XP100 with its 10" ventilated rib barrel.
The later models, such as the XP-100R, were made in .223 and .22-250
edit to add, there may have been custom shop models produced in .221, but I don't think any of the production guns were so made.
Last edited by tex_n_cal; 01/27/16.