I have formed 256 brass from 357 in one pass, but a pass with an intermediate die works a little better. I think I may have a bench-made intermediate die, let me check and if I still have it then I'll lend it to you.
I used Starline brass and it's too hard to neck down smoothly without annealing first. Actually that's also true of any other brand as well but the Starline brass seems even harder than most. In any case(sorry about that!) you'll need to anneal to avoid premature brass loss and I also suggest a preliminary trim and/or chamfer if the neck ends are at all rough or uneven in the slightest degree.
Annealing is simple and easy but requires speed and a certain amount of manual dexterity on a case this small. I use a propane torch turned on full and I hold the case rim in my BARE FINGERS, turning the neck in the flame until the color change (1-2 seconds) and then INSTANTLY dropping the case onto a COTTON towel. Like I said, it requires speed but you will soon develop your own rhythm and technique. This is only one of many annealing methods, certainly not the best but perhaps the cheapest and fastest.
I have successfully necked down and used brass without annealing but neck splits soon become a problem from the work-hardening.
Let me check my 256 inventory, if I still have dies and/or brass then you're welcome to use them.
Regards, Joe
