Originally 30 grains of the propellant that the cartridge was originally loaded with.
The 1894 Winchester rifle was introduced at a time when Black Powder (or Black Gunpowder) was being rapidly replaced by so called "smokeless" or "nitro" propellants.
The generic method for naming calibers prior to the smokeless powder era was usually to state the nominal bore size in 1/100ths of an inch, followed by the nominal weight of the Black Powder charge that it was loaded with. Some common examples are .44 WCF (.44-40) .45-70 Government (a US Military cartridge). There are always commercial exceptions caused by marketing and advertising departments. The .45 Colt could have been called the 45-40. the .38 WCF (commonly called the .38-40) should have been called the .40-38, because it is .40 caliber propelled by a nominal charge of 38 grains of Black Powder.
With the coming of the smokeless propellant era, some cartridges were named as in the Black Powder era, only the powder was now one (and only one) of several commonly available early nitro powders that are not likely being made today. For example the .30 US or Army is commercially called the .30-40 Krag, but it was never a Black Powder cartridge. The same is true of the .30-30 Winchester and the .25-35 Winchester which is a basically a .30-30 necked down to accept a .25 caliber bullet.
Last edited by Hammerslagger; 06-30-2012 at 04:54 PM..
Reason: typos; clarify