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Consider a 30-30 rifle cartridge, which has been around since the early days of smokeless powder. The name designation has significance in that it refers to having a 30 caliber bullet on 30 grains of smokeless powder. The 30-40 Kraig military cartridge comes from the same naming convention, a 30 caliber bullet on 40 grains of smokeless powder. This naming convention is no longer used.Consider a 30-30 rifle cartridge, which has been around since the early days of smokeless powder. It was designed for smokeless powder and introduced in 1895. The name designation has significance in that it refers to having a 30 caliber bullet on 30 grains of smokeless powder. The 30-40 Kraig military cartridge comes from the same naming convention, a 30 caliber bullet on 40 grains of smokeless powder. This naming convention is no longer used. With the 38 Winchester CF, formerly known as the 38-40, the two first numbers (.38) indicate the caliber and the (.40) indicate grains of gunpowder with which it was loaded originally. Such nomenclature is not standardized and you should not expect to encounter all naming conventions based on the same idea.Black powder has a low energy density compared to modern "smokeless" powders, and thus to achieve high energy loadings, large amounts of black powder are needed with heavy projectiles. The unit of measurement called "grain" dates back to medieval Europe. The average mass of a single grain of wheat was the standard for the grain unit of mass. The term "troy grain" was based on the average mass of a single grain of barley. The grain is used to measure the mass of bullets, gunpowder, smokeless powder, and preformed gold foil; it is the measure used by the balances used in handloading; bullets are measured in increments of one grain, gunpowder in increments of 0.1 grains.Black powder has a low energy density compared to modern "smokeless" powders, and thus to achieve high energy loadings, large amounts of black powder are needed with heavy projectiles.