When the duplex loads were new I read something you might want to think about. It was said that a layer of small shot over large shot (or vice-verso) is OK, but mixing shot is not. I think the main reason was that the small shot fills in the voids between the larger shot so the same volume of weighs more and increases pressure. Also, I think the shot charge is not as able to "squish down" as much like buffered loads which also increases pressure. Thirdly, I've read that one should never experiment around with shot shell loads without a pressure barrel because sometimes what seems like a small change from a given load with measured pressure data results in a large pressure increase.
