I read an article somewhere, about using IMR3031 as the duplex smokeless loading.
I tried that in my .50/70, 1859 barrel, lined and chambered in 1867 under contract with Sharps and the US Gvt. for the model 1868 Sharps rifle.
I put this barrel on my Italian cast 4140 action way back in the early 80's and tested it with straight and duplex loading.
With 10gr. of 3031 and 55gr. of 1F black powder, I achieved 3", 5-shot groups at 100 yards, along with the normal clean burning of the duplex load. IIRC, the best accuracy I got with straight black powder was 5 to 6".
The article's reasoning using IMR3031, was reduced pressure compared to using a faster smokeless powder like 4759, the then champion of duplex loading.
Too, using 3031 did everything the faster powders did, and shot just as well.
As a lower pressure smokeless, it's worth a try if that is a consideration. 10gr. was suggested for a 70gr. case. The .43 Mauser is a 77gr. capacity case, iirc.