Introduced in the Model 1873 rifle, the .44-40 Winchester was once to the deer hunter what the .30-30eventually became. Some cartridge historians have even said that the .44-40 killed so many deer that the .30-30had been around for many years before it accounted for an equal amount of venison. In the old high velocity factory loading, the .44-40 churned up over 1800 fps with a 200 grain bullet which compares rather closely to the performance of the .44 Remington Magnumin Marlin's Model 94 carbine. Correct bullet diameter for the .44-40 is .427", but some rifles in this caliber have barrel groove diameters large enough to take more readily available .429" bullets. However, since bore and groove diameters of .44-40 rifles vary considerably, it is best to slug the bore or make a Cerrosafe casting of it to determine true dimensions. 

This text is based on information from “Cartridges of the World”, Hodgdon reloading manual, the cartridge designer and/or own resources.