From The Manufacturer/Wikipedia/People Smarter Than Us:
Winchester’s 1873 rifle was the “Rifle that Won the West.” It was chambered in a new, more powerful cartridge, the .44-40—a .44 caliber bullet, propelled by 40 grains of black powder. However, a factor that really insured the success of the 1873 Winchester rifles was that Colt quickly offered its Peacemaker chambered in .44-40, as well. A cowboy could have both a lever-action rifle and his six shooter, and needed only one cartridge belt for both.