In 1873, Winchester introduced the gun that would be known as “The Gun that Won the West”—the Winchester 1873 lever action rifle, chambered in .44 WCF (Winchester Center Fire.) The same year, Colt introduced the 1873 Colt SAA revolver in Colt .45 and the US Army introduced the .45-70 Government.
Colt, realizing that western settlers wouldn’t want to carry different types of ammunition for their rifles and side arms, decided to chamber their 1873 SAA revolvers in .44 WCF—though they renamed the .44 WCF to .44-40 (.44 caliber and 40 gains of black powder) in order to prevent giving Winchester free advertising.
Winchester never reciprocated by chambering any of its guns in Colt .45.
Today, most lever gun reproductions are chambered in the Colt .45 for two reasons. First, the rims on modern Colt .45s are stronger than their 19th Century counterparts; and second, .44-40 Winchester ammunition is rather scarce.
If you have considered buying or reloading the .44-40 Win, here are some helpful recommendations from my experience.