I have owned Fireballs. Less velocity than a 223 and the same noise level. On the other hand 223 brass is dirt cheap and everywhere.
The .221 Fireball was designed to be used from 10" barrels. It, literally, holds all the powder you can burn in a 10" barrel. And a 10" barrel is what Remington made in the XP-100 pistol.
I have one, and a .222 Rem 10" for my Contender. The .222 holds a few more gr of powder, doesn't go any faster.
Now, move out to 14" or more, and the .222 and .223 get a benefit, being able to use more of the powder they hold in the longer barrel length, and so are faster than the .221 from longer barrels.
As far as the brass goes, yes .223 brass is everywhere, and you can make .221 FROM it. Though that's not an issue for me, I have a couple hundred factory Fireball cases and that's a long time supply for me.