I am not a big fan of BP. Hobie's article was good but not very much new or in depth info there. Mind you I am not advocating hot rodding the newer and stronger 1876 Reproductions here. But where it is suggested nothing faster than 2400 and 100% powder loads I agree and suggest looking even farther. Nothing wrong with using Smokeless and easily duplicating the old BP velocities either
in the new guns. I have no interest in duplicating BP loads or using fast burning smokeless powders to partially fill 45-75 cases and risk disaster.
IMR 4198 and 3031, H 4895 and 4831 all seem appropriate from what I see and other's experience. And all are much, much slower than 2400. And with the right powder you can have 100% of the case volume filled with bullet and powder. Just as the original cartridge design intended. Now we have new rifles made from modern ballistic quality steel and tight tolerances to finally see how capable the new 1876s are.
In Hornady's 9th Edition there are 146 powders listed. Fast to slow..#1 being the fastest burning. #146 the slowest
Bulleye is #2. Alliant 2400 is #46, 4198 #64, H 4198 #65, 3031 is 69, H 4895 #79, IMR 4320 # 95, H4831 #125, H4831SC #126, Pyrodex CTG #144.
The 45-75 is (the Uberti chamber makes it even closer) a short version of the .45 Alaskan wild cat. Which is .348 Win brass necked up to 45. .50 Alaskan is a .348 necked up to .50 cal. Both mimic the earlier '76 WCF loads. Difference is both are loaded hot with heavy for caliber bullets and high pressures in a 1886 or '71 Winchester. Loaded and shot lots of both. Way too hot for a '76 and not needed for most anything short of a big bear charge. ( even then factory .45 or .50 BP vel. would likely do just as well) Also loaded .505 in a Rigby case and a 505 Gibbs. 4320 and 4831 both work well with big cases and big bullets. And they seem to work pretty good in the 45-75. Nice that it is clearly impossible to double charge your '76 using the slowest of these powders.
Group below was shot today off a sand bag rest @ 50 yards using 58gr. of 4831SC with a 350gr lead Oregon trail bullet, magnum primers and a tight crimp on the groove. There is getting to be some recoil you might notice with my 20" gun. Gun and load clearly can shoot better than I can. Same load with a similar 300gr bullet was more fun to shoot but left some unburnt powder.