baldie--Fire forming the .222 Mag brass is a wee bit time consuming, but if you find a load that has decent accuracy, then you can use it in the field instead just wasting the powder, primers, and bullets trying to fire form the brass. I did find a fire forming load that I would take out prairie dog shooting if necessary.
My cache of .204 Ruger brass is fairly large now. I have 600 WW casings and 320 Hornady casings. I did fire form about 50 of the .222 Mag casings and have another 40 ready to be primed, loaded with powder and bullet seated, but will hold off on those until winter I think.
I think you can get by quite nicely with the Hornady dies. That was the only company that had .204 Ruger dies available when I first started reloading for that rifle early last year. I bought the full length sizer, neck sizer, and seating die from Hornady. Later, when Redding started making dies for the .204 Ruger, I picked up the Redding Type S bushing neck die, and when Forster made their Ultra Micrometer seating die for the .204 Ruger, I purchased one of those. I still use the Hornady neck sizing die to partially neck size my .222 Mag casings so they crush fit into the chamber.
Take your time and make sure you have the little .224 portion of the neck out far enough so you have to use some force to push the bolt forward and downward to chamber the casings you are going to fire form. As to the powder charge to use, I can't really say what will work in your rifle. Initially, I used 27.0 gr. of H4895 and the 40 gr. V-Max bullet with the bullets seated approximately .007" into the lands. It IS NOT NECESSARY to seat the bullet into the lands because the little .224 part of the casing neck is going to push the back of the casing against the bolt face and prevent a headspace problem. Those casings formed nicely. I got an average of 3,701 fps at 12' 9" from the muzzle, but the accuracy wasn't super good.
I increased the load to 28.0 gr. for the next fire forming bunch and that was TOO MUCH powder. The primers showed signs of flattening. My average velocity for 25 rounds fired with this load was 3,867 at 12' 6" from the muzzle. That, IMHO, was whizzing those bullets out there pretty fast. The groups were more like a shotgun pattern. The smallest 5-shot group measured 1.223" and the largest measured 1.704".
I was running out of 40 gr. bullets so for my next fire forming stint I used 28.0 gr. of H4895 with the 32 gr. V-Max bullet. As stated in that link I gave you, those groups were fairly decent for a fire forming load and can be used in the field if the distances aren't too great. The velocity was acceptable too.
I hope you have good luck with the fire forming, if you have to go that route. Take your time and maybe you'll come up with a fire forming load you can use in the field like I did.