RE: whats the difference?
The .223 is about as potent at 100 yards as the .222 is at the muzzle. They're both legal for deer here in Texas, but, you really need to be EXTREMELY selective with where you place the bullet. You should check whether they are even legal where you hunt. Some states mandate a .243 caliber minimum. I would consider the .223 a marginal deer rifle out to 100 yards, and the .222, obviously, even less appropriate.
The .17 HMR is absolutely NOT interchangeable in a .22.