44/40. You have to like it to put up with it.
Remington, Winchester, Hornady and Magtech all make it. RP, WW and Hornady make it with JSP bullets. RP, WW and CBC (Magtech) make it with lead ("cowboy loads").
It's expensive as hell. Between 30 and 50 dollars a box, before ammo prices went crazy. Always hard to find, and now, probably, even harder, since the machinery used to make it is (most likely) also used for 44 magnum, and the ammo companies can sell more of that.
Factory 44 Special is loaded lighter, while factory 44 Magnum is loaded heavier. The specials can be loaded to 44/40 balllistics.
It's been used, effectively, to kill deer, bear and people since 1873.
Reloading it can be a pain. It is a bottleneck case, so it requires bullet lube (they don't make carbide sizing dies for bottleneck cases). The mouth is extremely thin, as compared to 44 magnum or 45 colt. Stepping on a case can damage it beyond repair. A slightly off-center case going into the die will destroy the case.
It can be reloaded quite cheaply. Store-bought 200 grain lead bullets can be had for less than a dime. Powder runs about 2 1/2 cents, while primers are 3 to 4. 15 cents a round is 7.50 a box. Cast your own bullets and the price drops. Depending on the manufacturer, you will either have a .427 barrel, or a .429 barrel. Original guns were 427, but many makers (Ruger, especially), nowadays, use 44 magnum barrel stock (since they already have it) and that's .429.
Some rifles are picky about the brass. My 73 does not like RP or CBC brass. My 92s eat anything I feed them. I think it's the rim thickness.