Willi, if you aren't going to shoot black you had better slug the barrel. Mine was .446 and it would never shoot the .439 bullets with smokeless, only with black where they could bump up. I had a 446110 HP that I sized .446 and they were fine with 28 grains of 4759 with no filler. I suppose you could paper patch the .439s with thin paper and get them to work but I'm leary of using any fillers. I also had an Argentine rolling block in 43 Spanish that was 446 and I had Bernie Rowles at Old West Bullet Molds make me a mold for a heavier bullet in 446.

Yes, definitely safe to shoot but the target was also safe! You have to watch the 43 Mausers also. I have one that is .446 and another that is .439.

Thanks gentlemen. I had no intention of compressing a smokeless load in these large casings, and while I had not found anything regarding using a filler with Spanish 43, I did for the Mauser 43 load. So, I just wanted to confirm.

I am new to the SA 19th Century black powder to smokeless transition, so I am proceeding cautiously.

I do plan on using BP for my Mauser 71.

I will slug the barrels, but a quick muzzle groove measurement yielded .438 on the Peabody, .443 on the 71, and .438 on the 71/84.

For safe loads of smokeless in old large capacity cases (in my case 20 odd grains of 5744 in 45-70 cartridges) I did not want to use any kind of fillers.  So I followed Charlie Dells advice of using a light card or cork wad held just slightly above the surface of the powder---NOT down on it and NOT compressing it.
As a wad seater I used a very fine threaded metric bolt that was a slip fit in the case and two nuts to lock the depth.  I started by seating 3 cork wads very lightly on the powders surface, then locked the nuts in place removed the wads, and then used just one wad.  It was enough to hold the powder charge a proper position for consistent ignition while not compressing it.   
  However that method might not work well in most bottlenecked cases.  

  Another option is special cases.  At one time I had RMC turn a small batch of custom cases for a non-standard 11mm BN "mauser" Euro-stalking rifle.  The outside of the cases matched the chamber cast properly but the internal powder chamber of the case was straight-walled from the normally thin case mouth to the primer flash hole.  The cases are thick walled like everlasting cases.  This will reduce the capacity for BP loads of course; but for 5744 and 4759 type powders they work well. Its a simple break open stalking rifle so I have never tried using it in a match for target accuracy.

Thought I would provide a quick status of what I did a few weeks ago.

Slugged the barrel and it measured .443". Tried the 370gr .439 bullet knowing what to expect, but wanted to see it myself. 1 hit out of 5 on paper at 50 yards. Keyhole. 4 hits out of 5 on paper at 25 yards. All keyholes. Casing would need to be resized after firing as .439 bullet just falls into casing. Bertram brass. Load was 20grs of 5744.

Next I thought I would try some Jamison brass with some .446" bullets, both the 340 and 385grs. Had to use a Mauser 43 ball expander to open up the casing, and only belled he mouth slightly. Had to apply a slight crimp to chamber the round. Both bullets shot about 12" high at 100yds, with 5-shot groups between 2.5" and 2.75". Will not have to resize the brass after firing as the .446 just barely starts to seat by hand. Also used a load of 20grs of 5744.

Next will be to try some different loads and chrono them. Will run the Bertram brass through the Mauser 43 ball expander. Disappointed with the high impact, but doubt reducing the load will get me near where I need to go.