I have a Sako L461 in 222Mag, for about 25 years now.

I use 223 load data as a guide, and work up from there.

In general, 222Mag brass has about 5% more case capacity than the 223. I mainly use Remington brass, but also have a supply of Herter's, which is supposed to be made by Norma. The Herters brass is much more consistent in weight than the Remington. Nosler has made brass and loaded ammo, in the past.