The .38-40 Winchester is actually a .40 caliber (10 mm) cartridge shooting .401″ (10.2 mm) caliber bullets. The cartridge was introduced by Winchester in 1874 and is derived from their .44-40 Winchester. This cartridge was introduced for rifles, but in its reintroduction for Cowboy Action Shooting it has seen some popularity as a revolver cartridge. It is not particularly well suited to hunting larger game, but it was popular when it was introduced, along with the previous .44-40 Winchester, for deer hunting. It can be used successfully on smaller game animals, and for self-defense. The 38-40 is based on the 44-40 necked down to .40 caliber.
The 38-40 was first offered in the Winchester 1873 rifle. Later, Colt, Remington and Marlin also chambered rifles for it. It is most often found chambered in the Colt SAA revolver. This old cartridge should only be loaded with Black Powder or Black Powder Substitutes. It does have loadings listed for smokeless powders.