>>639555585.7 is very much a pistol round and not even close to 5.56 other than the bullet diameter. .221 Fireball is just barely longer than 5.7, you could load it to 5.7 length if you wanted although you'd lose a lot of range from needing to use stubby pistol-style bullets, but you'd have far more muzzle energy, much closer to 5.56 than 5.7.
Also, you could get even more energy by using a super short loading of .300 BLK rather than .221 Fireball (it's the same case just necked up) although you'd fuck up the external ballistics even more. But if you don't plan to use it at ranges beyond 100 yards I think it's a great option. Just load some .30-30 semi-wadcutter bullets in .300 BLK cases and build a FiveseveN-sized Boberg. You could fit a 7" barrel in a full size duty pistol, that would absolutely give you 5.56-level power.