I've made Fireball cases from 223. I used a Fireball sizing die and just took extremely small steps until the case bottomed out in the die. Cut the case with a hacksaw, chamfered the neck inside and out, loaded them up and shot them. No problem, except I wouldn't want to make a career out of it. Plus, when I first bought my XP100 (at least 20 years ago) I bought 500 rnds of new brass. It still sits on the shelf, untouched. If I had it to over again, I'd take the route CZkid described.
I see the 20 Vartarg as the next center fire for me.
I see the 20 Vartarg as the next center fire for me.