What's wrong with reloading Black Powder Cartridges?
They go "BANG!", they push gigantic chunks of lead down the range and they can be utterly devastating on a gallon jug of water. FUN!!!!
If you don't like the FACT that Back Powder residue just LOVES to eat your brass, just as fast as it can, you can do several things. The more complicated method is to drop your brass into a bucket of water, then boil it out after the range session, dry ad start weighing your charges and reloading with just the right amount of compression and all the rest. This gives you a dead duplicate of the original ammo.
OR you can do what us Lazy Sods do...... and duplicate the COMMERCIAL loads which were available here in Canada up until 1968. These were SMOKELESS loads and they were a precise duplicate of the original loads in every way except that there was no smoke, no brass getting eaten for lunch..... and no high pressure, which is what will wreck one of these fine old rifles in about 1/1000 of a second.
The .43 Mauser was actually, by the old American scales, a .44-77-386. The BULLET was .446" diameter (for the .433" bore) and weighed 386 grains. The CHARGE was 77 grains of Black.
Now, there still exists an efficient and available Smokeless-for-Black powder, the last descendant of the ancient Bulk powders. It is a Sporting Rifle powder, SR-4759 and it has been out of production and back into production and out again and in again more times than I can count since I bought my first tin. Right now, it is IN production and it is wonderful. The FORMULA is simple: for any Back powder rifle cartridge, load normally except that you substitute SR-4759 for your Black Powder charge in a load of 38%-by-weight-of-Black. Thus, for a .44-77-386 you would get the correct casings (.43 Mauser in this example) and load your 386-grain cast lead bullet with (77x.38=) 29.3 grains of SR-4759. The charge will be very bulky, you don't need fillers or wads: just dump it in, seat a slug and head for the range.
Your RESULTS will be a perfect duplicate of the original German military load..... except for smoke and corrosion. You even get the same pressure levels, so no problems. And the stuff is bulky enough that you REALLY have to screw up to double-charge and not notice it.
SR-4759 is the PERFECT smokeless powder for just about ALL those wonderful old antique Black-powder rifles. It works fine in Winchesters and Mausers and Vetterlis and dozens of others although, to be honest, I DO shoot my Sniders with Black: some of those old iron barrels make me just a bit leery, although ICI did load Martini ammo with smokeless for many years..... but even they stayed with Black for Sniders.
Hope this helps.
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