I suppose you could run just the neck of a Herret case into a .357 die to size it, but how are you going to keep the body of the case concentric with the neck? The way you want to set it up is to get the full length sizer, some brass that has been fired in your chamber, and the barrel with the extractor removed. Use feeler guages to see how much gap you have between the barrel and the frame. Write it down. Take the barrel off the frame, and make sure the chamber is absolutely clean. Brass too. Back off your full length die a turn or so, and size a case. Drop it in the chamber and see how far it sticks out. Now adjust your die so the case sticks up past the end of the barrel about .002"-.003" less than the barrel/frame gap. Now load and shoot some. Check again with the same cases. If they are still in the same place, your die is set. Don't move it again, any other lack of accuracy is related to something else, not your sizer die.
