Professor Marvel has given the best data on the differences between the .44-40 and the .44 Russian/Special/Magnum family of cartridges. (The basic case for the .44 Russian/Special/Magnum family is the same, differing only in case length and the allowable pressures, based on the strength of the guns. The longer cases were ostensibly designed so that the hotter cartridges couldn't be chambered in the guns for which the Russian and Special rounds were originally intended. The older, shorter rounds can certainly be fired from .44 Magnum guns. Whether rifles chambered for the longer rounds will feed reliably is another matter.)
While I have heard of the "family" cartridges being fired in .44-40 chambers, it is a BAD IDEA! The likelihood of a split case is high!
Interestingly, while the .44 Special family uses a nominal .429-.430" bullet, I've measured 2nd Gen. Colts chambered in .44 Special with .427" barrels! As to the matter of bullet diameters in the .44-40 and the "family", it is true that the original .44-40 bullets were nominally .427" diameter, I have measured throats in everything from original Colt's Frontier Six Shooter at .425" with a .427" barrel, to .425" throats in Old Model Ruger Vaqueros with .429" barrels (in the 55-XXXXX to 56-XXXXX serial range, before Ruger went to .429" throats just prior to discontinuing guns in .44-40), to original Winchester '73's with .434" groove dia. barrels!!
Most modern Italian-made guns use .429" barrels. I haven't measured enough to know how they are throating the revolver cylinders.
The result there is slug the barrel and the chamber throats, and then decide the bullet diameter that gives the best accuracy. But DO NOT shoot .44 Specials in .44-40 chambers!