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In Paul a matthews book ‘ loading the black powder rifle cartridge’ he suggest starting with a bore size bullet and PP your favorite paper. Or patch up to bore size. I am going to try these bullets. Do a search on forum- black powder cartridge for other help for 43 Mauser. I am going to try a straight load of 3fg now too. Someone said that 43 Mauser is in the transition size between 2fg and 3fg- a bullet master.
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I have a Mauserbuchsen, a single shot based on the 1871 action, but scaled down and using the 9.5x47R cartridge, derived from the 43 Mauser case.
I paper patch for mine. It has relatively shallow grooves, .375” vs lands at .368” and a progressive twist 1:14. Reading Mathews (Paper Jacket) again and Col Ned Roberts and both make comments about European military rifling generally being very deep and the paper jacketed soft lead bullet much more prevalent than the US, where the military went from grease grooved naked lead to cupronickel jackets. The Europeans did fine with deep grooves, soft lead, and paper patches. As suggested above, patching to land and a wee bit is the way to go. Soft lead, I prefer pure lead, will obturate and between the patch and fouling, you should get a decent gas seal.
As to powder, I use 1.5 Fg Old Eynsford in my .375”. I think 3Fffg will probably not give best results in your rifle, but with the component issues these days, if 3F is all you’ve got, you can probably make it work, but, in my experience, the larger the bore, the more sensitive it is to variations in the charge of finer powders, ie a few grains of 3 F will make a bigger difference than a few grains of 1.5 Fg. So work up loads carefully for best results.