Cartridges began to be used in breech loading rifles in the 1860s (Civil War period). They consisted of a case, some type of powder, a primer, and a bullet- which was much more convenient and quicker than muzzle loading with separate components. As others have stated, this is when the barrels began to be stamped with the caliber and "CTG" to signify that this firearm used cartridges (of a particular caliber). CTG Revolvers appeared shortly after rifles, and their barrels were similarly marked. S&W and other revolver manufacturers continued to use that nomenclature on their barrels for some time. I'm not sure exactly when S&W dropped "CTG" from their barrel stamps, but it probably began in the 1950s because I have .357 Magnums and .44 Magnums that do not have the "CTG" stamp (they just have the caliber), and none of my -2 models have it. Although I think S&W continued to use it with the Highway Patrolman and Model 28s.