Your brass is not formed until you do so, and this requires fire-forming in your chamber (your best die).
But it is simple enough to know what you will fire-form brass to, and what you want to size where & how much. This is where the sizing reamer comes from (those understandings). Any die maker should be willing to trust that you know what you're doing in designing the sizing reamer. He can make it using the reamer/print, and could not possibly be held accountable if your plan didn't work out. It's YOUR plan, it's just a die, and you could simply learn & adjust until right.
Nothing wrong with using FF'd brass, if again there was understanding behind it. The greater to clearances in forming, the greater the change to spring back of the brass, and the less predictable sizing needs will be. So until knowing the outcome there is still an element of chance to it.
I've had good fortune with both, but then my clearances were small and my ammo turned out exactly matching earliest design. So did my estimated H20 capacities, and QL generated loads.
But it is simple enough to know what you will fire-form brass to, and what you want to size where & how much. This is where the sizing reamer comes from (those understandings). Any die maker should be willing to trust that you know what you're doing in designing the sizing reamer. He can make it using the reamer/print, and could not possibly be held accountable if your plan didn't work out. It's YOUR plan, it's just a die, and you could simply learn & adjust until right.
Nothing wrong with using FF'd brass, if again there was understanding behind it. The greater to clearances in forming, the greater the change to spring back of the brass, and the less predictable sizing needs will be. So until knowing the outcome there is still an element of chance to it.
I've had good fortune with both, but then my clearances were small and my ammo turned out exactly matching earliest design. So did my estimated H20 capacities, and QL generated loads.