Daryl,
My .577 is a converted 10 bore and uses the magtec 28 gauge brass. They work really well and don't cost much at all. We annealed them befor using, and are holding up very well. The 577 is loaded fairly mild at intially 80 grains of FFg, so with the increase in volumn of the 20 gauge, I would think 100 grains would be sufficient and not too heavy on recoil. I will have custom molds made for probably 680 to 720 as I don't have the need for the standard 900 grainer.I'll look for some back article information on Handloader to see if I can find that article.
Thanks guys,
TC