The problem with 44-40 is that it's more on the pricey side and not nearly as common. With no reloading capability at the moment I'm stuck with buying commercial ammo. Plus the added convenience of having the same caliber for both pistols and rifle. As far as black powder, what makes the 44-40 preform above the 45 Colt? Also, how much of a task is it to clean a lever action rifle using black powder? I fully disassembled my 94 once and it wasn't fun getting it back together. Not on the level of getting an old H&R revolver back in place but tricky nonetheless.
There are some sources for cowboy factory ammo that can supply .44-40 at reasonable prices. The .44-40 (aka .44 WCF was developed by Winchester and introduced with the 1873 rifle; it was one of the first commercially available centerfire cartridges. It was, of course, loaded with black powder at that time. Colt introduced the Single Action Army revolver the same year, chambered in .45 Colt but in the 19th Century that was a revolver cartridge-no rifles were made in that chambering.
The .44 WCF, like its progeny the .38 WCF (aka .38-40, which despite its name is a .40 caliber round) and the .32-20, are slightly bottlenecked rounds and the case mouth is fairly thin compared to the .45 Colt. These features cause the case, upon ignition, to make a good seal in the chamber so that all the gases and fouling pretty much go down the barrel, with little or none flowing back through the chamber into the action.
Not all rifles are the same. A friend of mine who is a big black powder cartridge shooter likes the Marlin 1894 for black powder shooting because you can remove the lever screw and pull the bolt out of the back of the receiver, making cleaning very easy. He shot .45 Colt in his revolvers and rifles for years. When he finally bought a Uberti 1873 in .44-40 he pronounced it "very good" for black powder. The 1873 is not quite as easy to disassemble as the Marlin but it's much less complicated than the 1892. Of course, he tells me that his action stays so clean shooting the .44-40 that he rarely has to take the rifle down.
Is Trail Boss the best powder to use for loading Cowboy Loads? I've seen it mentioned several times in various posts.