I am firing mine with a 12-bore shell cut back to 2 inches, 60 to 65 grains of Black powder and a 480-grain .690" hard-lead Ball in a standard modern shot-cup/wad assembly.
Mine was at the CGN "Meet & Greet" (see 2 threads in Milsurp forum) held here in Virden on Saturday. I fired it, Tinman204 fired it, TWOSTEAM fired her own and we all still are upright and walking around, so they can be shot safely.
Owing to the very short Breech Block and the distinct "drop" to chamber a cartridge, it can be difficult to wrestle a full-length 2-3/4 shell into the critter and would be a lot harder to get out once the shell has opened up, being that the empty casing has to go UP a ramp to eject. I experimented for an evening with mine after I got it, trying to ascertain what the limits of the action might be, and found that the longest shell you can get into the gun easily and get out again is about 2 inches total overall length, so that is how I loaded my shells.
COTW-6 gives the Charge in the original military round as 69 grains and the Bullet Weight as 555 grains and .740" diameter. There was a special Carbine loading which was actually HEAVIER, if that makes sense. MOONCOON's dummy round, shown above, is just about PERFECT. I WISH I could turn out one as nice!
The Zulu guns are what are left of the Tabatieres with which the Imperial Army of the Emperor Napoleon III was armed. They served during the Franco-Prussian War (1870 - 71) and were sold off as surplus near the end of the 19th Century. They went to Belgium and were converted into cheap shotguns. The Belgian gunmakers were so efficient in their work that today VERY few original Tabatiere rifles remain in existence. They are few, far between, and quite scarce and expensive enough that I will never be able to afford one..... so I got a Zulu and I fire Balls from it. Gawd help the poor T-Rex that gets too close!
Remember, these things ARE contemporaries of the Snider, the action is NOT as solid as a Snider action..... and they are made of SOFT BROWN IRON. In my opinion, these old things are fun with light charges of Black Powder but, if you are firing modern Smokeless-Powder ammunition in one, either you are VERY lucky..... or suicidal.