You would think this is a pretty obscure caliber, but at least two other forum members here shoot one. When I got my first 10.4 Vett, I did a lot of research to find the proper bullet. A friend in NJ told me RCBS made a dedicated 315 grain heeled bullet mold so I called them up. Long story short, I got the very last one they had. No more, they say, but since they have the cherry, I think there's a possibility.
I did a lot of experimenting with black powder subs since real BP is a little hard to find out here in the sticks. I had some Hodgdon Pyrodex CTG and had my best results with that. 777 also did well, but it's hotter than Pyrodex. There was another brand called shokeys gold, but I am really soured on that stuff! Anyway, here's what I've come up with so far using the dedicated hollow base bullet.
Barnes COTW shows the military loading was a 313 lead bullet with 77 grains of Fg BP for around 1255 fps.
With cases made from WW 348 and Std large rifle primers, I had the best accuracy results with 53.0 grains of Pyrodex CTG. Velocity averaged 1430fps.
Since that powder is hard to find ( I've heard it's discontinued) I also experimented with Triple Seven. 49.0 grains of that gave me the best group = 3 shots touching @ 100M and an average velocity of 1396 fps.
Now I'm working with Accurate Arms 5744. I called them, and they recommended charges of powder between 21.0 and 25.0 with a 315 grain bullet.
I tried 22.5 grains with magnum primers, and that load has accuracy potential. However, I noticed primers were very slightly deformed as if pressures were building so I've discontinued that one for now. Three shots went into 3 inches at 50M, and that's the best accuracy I've had with 5744.
Today, I backed off to 21.0 grains of 5744, but accuracy really suffered. Five shots only made a 12" group @ 100M.
5 shots averaged 1235 fps.
Like Wile E. Coyote, it's back to the drawing board. I'll try going back up in powder weight a little at a time and see how accuracy does.
While I don't have a lot to report, this will at least give other experimenters something to consider with their own efforts. Another forum member has written me with his very favorable results using regular 300 grain 44 bullets sized down, and he should be posting that before too long.
Still nothing solid to report on 43 Dutch Beaumont. I ordered the 345 grain Lee 45 cal mold and a .457 Lee sizer today. More on that caliber later. SW
***DISCLAIMER*** All the standard caveats apply here! your mileage may vary, etc. & so forth. Proceed with caution, Friends!