About duplex loading - many years ago I read an article about using IMR3031 as the smokeless powder in duplex loading, so I tested that method.

In the .45 2.1" Hoch bl. and then my .50 Alaskan McGowen bl. (after removing the .45 barrel) on the rolling block (at that time - 1980's)

I used 10 and 15gr. respectively of 3031, the rest of the space filled with 2f GOEX - but 10gr. & 15 less than the normal BP loads.

Accuracy was much improved and as Wayne noted and they shot quite cleanly.

As to the accuracy exactly, the accuracy with the IMR3031 in the duplex loading was better than when using SR4759 or Unique, the other smokeless powders I had used in duplex loads since the mid 70's.