Okay, I checked reamer dimensions for the .221 fireball and the .222 remington. It looks like a .222 remington reamer will clean up either a short-chambered or full-depth .221 Fireball chamber correctly. See for yourself at
www.clymerreamers.comI would still strongly advise against attempting to do the rechambering by hand. Manual chamber reaming is okay because little material (0.50") is removed to achieve proper headspace. Trying to deepen a chamber shoulder by 0.19 inch manually is asking for trouble because any sideways pressure will result in the chamber becoming oversized or eggshaped. Rechambering (including to "improved" or blownout cases) is normally done on a lathe using steady rest and tailstock to apply reamer feed. Extra care in measurements, alignment, lubrication, and chip removal is what gives perfect chambers.
Anyone experienced in the gun repair business can tell you horror stories about damaged chambers, hard extraction, and ruined brass from amateur attempts to "improve" rifle chambers. I've seen a few disasters caused by running standard machine reamers using an electric drill. My favorite is from the late Frank de Haas when he was an NRA technical editor and received a Japanese 6.5mm Arisaka rifle that was rechambered to .30-06. The owner complained that it "kicked too hard" and was difficult to extract, even though he had killed a couple deer with it. Yeah we can imagine how it must have kicked, firing .308" bullets through a .264" bore!!
Anyway, enough said. The CORRECT way to rechamber is on a lathe, if you want maximum accuracy and performance. but it's your rifle, and you are certainly free to do whatever you want. Let us know how it works out for you.
John