Cutting Through Caliber Clutter

Names, numbers and letters stamped on the cartridge case are…

Names, numbers and letters stamped on the cartridge case are there mainly to ensure you get the right ammo in the right rifle. Rarely are the names a precise indication of the cartridge’s caliber or power level.

You would think that something as simple as naming a cartridge is easy. Truth is, manufacturers have been muddling this up since day one. It’s to the point now that using numbers to reflect the bullet’s actual caliber is no longer viable.

My grandfather took me to visit an old farmer many years ago. An old Savage lever-action was leaning in the corner of his bedroom. I asked the farmer what caliber it was, and he said “three-hundred,” like there was only one .300 cartridge in the world. The rifle was of course chambered for the .300 Savage, but that is not the only cartridge that bears a “three-hundred” in its name. Note that the caliber bullet that the .300 Savage uses is actually .308.

Generally, the bullet’s diameter is indicated in a cartridge’s name, for example the .308 Winchester. But that has not always been the case. The .275 Rigby is actually a 7mm Mauser cartridge adopted by John Rigby & Company in 1907 and uses a bullet that has a diameter of 0.284 inches. And 7 millimeters is actually equivalent to 0.275 inches, which is very close to the land-to-land diameter inside the barrel of the rifle. For what it’s worth, bullet diameter is and should be equivalent to the groove-to-groove diameter inside a rifle barrel.

All three of these cartridges are known as a .300. They are very different from each other and were introduced many years apart. The .300 Savage was introduced in 1920 (left), the .300 Winchester Magnum in 1963 (right) and the .300 Winchester Short Magnum, or WSM, in 2000 (center).

Most .308 caliber rifles are referred to as .30 caliber rifles, but this too can get confusing. The .307 Winchester is a rimmed version of the .308 Winchester, loaded to slightly lower pressures. The .307 Winchester was designed to work in Winchester 94 and Marlin 336 lever-action rifles and does indeed use a .308 caliber bullet. Winchester could have named it the .308 Winchester Rimmed Low Pressure, but then who would have wanted one. The same situation occurred with the .356 Winchester, which was nothing more than a rimmed version of the .358 Winchester.

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