User Gamsek sent me some 12 gauge Mihec RB:s to try, thanks a lot!
My good success with similar 20 gauge loadings got me interested,I have used Lee RB:s for that.
Right now I'm loading,this is very easy: Take a factory trap round,cut the crimp,pour out the shot,put in a felt wad and a round ball,roll crimp tight. Done! (A nitro card under the ball might be better but I don't have any. I use 13 mm cork for 20 gauge.)
A range report will follow.