I have 2 43 Mausers that I played with 25 years ago or so. One is a hammer combo that except for having a 16 gauge shotgun barrel alongside looks like yours. Its groove diameter is .439 which was apparently common for them as Lyman specified their mould 439186 (385 grains) for 11mm Mauser. RCBS has a similar mould at 370 grains. At the time I was working up some smokeless loads for it (with only a little brass on hand and shooting 2 shot groups at 50 yards) the best load was 30 grains of 4198 giving a 3/4 inch group falling 2 inches high and 2 inches left of the sighting point. At 100 yards, a lighter load of 28 grains did well. The bullet was seated long to get it closer to the rifling and lower the pressure. I did a ladder test with Reloader 7 at 100 that suggested that 24 to 25 grains of that powder might work well. 27 and 29 grains of 5744 did not do well but with the 29 grains 3 of the 5 shots at 100 were in 4.5 inches and I noted to try increasing that load. My other rifle is a 71 Mauser carbine and it has the other common size of .446. Besides the 439186 (which casts larger) I have Lyman mould 446110 but the hollow point version which casts at 340 grains (the more common non hollow point - therefore heavier - might be the bullet for you). I had only tried it with the old and no longer made 4759 which gave me .6 to 1 inch groups at 50 yards. As with most military rifles the lowest sight setting shot high at 100 so I modified the sights to shoot lower. With black powder at 100 I tried 84 and 86 grains but noted that I might try 80 to 82. The 84 grains with the bullet seated out gave me a 5 shot group at 100 of 4.3x2.6 inches slightly low and to the right of my aiming point. I only shoot cast bullets. I only neck size and usually not the entire neck but just the part of it that holds the bullet. These tests were done long enough ago that I can't say what black powder I used. For the last 20 years or so I have been shooting BPCR and using excellent Swiss powder in a 1 1/2 grade. Loading black powder in a target situation is not a science but an art. There are many variables to be explored. If, with the bullet seated out to approach the lands and with the powder load that you want to try with a wad seated on the powder with just a little compression, you find there would be a gap between the wad and the base of the bullet, you can add more wads or use some amaranth seed on top of the powder before adding the wad. I have to do this with a case that would hold 115 grains of powder and I only want to shoot 85 grains in. To maintain accuracy for rapidly repeating shoots in BPCR, I originally used a blow tube between shots but soon switched to a slightly damp patch (1 NAPA water soluble cutting oil to 6 water) run down the barrel on a nylon brush followed by a quick pass of a bore swab to the chamber (a wet chamber can cause case separation). For hunting, you should be able to get several shots off before accuracy drops too much. The other trick for hunting (not legal for BPCR) is to replace a few percent (less than 10%) of the weight of the BP with a smokeless powder loaded first into the case. This is called a duplex load and will cause the BP to burn much cleaner. Writing this has got me wanting to play with these rifles again so thanks for that.
