My Uberti 1866 will not take a bullet larger than .428. .429's jam in the throat. I use the same formula as Driftwood, as much 2 or 3F as she'll hold with a Big Lube 200 grain bullet sized to .428.
Howdy Again
At last count I have five rifles chambered for 44-40 and two revolvers. I have slugged the bore on all the rifles, however I cannot get an accurate reading of the rifling groove diameter of the two revolvers, as they have an odd number of grooves.
Anyway:
Marlin Model 1894 from 1895 - .427
Winchester Model 1892 from 1897 - .427
Uberti 1873 from the 1980s - .427
Uberti 1860 Henry, from about 2008 - .429
Winchester Model 1892 from 1918 - .429
I have listed the rifles in the order I obtained them, not in the order they were made. As can be seen, the first three all had groove diameters of .427, including the fairly recent Uberti 1873. I could shoot .427 or .428 bullets in these rifles, however the Uberti 1873, which was my main match Cowboy rifle for some time, had a tight chamber, and if I used .429 bullets the case mouths would expand enough that the rounds would not chamber completely. So I was shooting .427 bullets exclusively.
When I purchased my Henry about ten years ago, I slugged the bore and it turned out that it had a .429 groove diameter, much like most of the modern Uberti 44-40 firearms. So at that point, I comprimised on .428 for the diameter of bullets for all my 44-40 rifles. .428 works fine in everything, and works fine in my 44-40 Merwin Hulbert and Colt New Service, which are the two revolvers I own that are chambered for 44-40.