Caliber and cartridge are related terms, but they don’t mean the same thing.
Caliber refers to the diameter of the bullet or bore. For example, .30-caliber bullets are roughly .30 inches in diameter.
A cartridge is the assembled ammunition. Cartridges are often casually called calibers in the shooting industry, but a cartridge is the complete ammunition that fits a specific rifle chambering. Common cartridges include 6.5 Creedmoor, .308 Winchester, .30-06 Springfield, and .243 Winchester.
Many cartridges use the same caliber bullet. A .308 Winchester, .30-06 Springfield, and .300 Winchester Magnum all use bullets that are the same diameter, or caliber, but each cartridge is completely different.
The easiest way to remember it is:
Caliber describes bullet diameter. Cartridge describes the complete ammunition/chambering.
If your rifle is marked 6.5 Creedmoor, .308 Winchester, or .270 Winchester, buy ammunition for that exact cartridge—not simply ammunition with a bullet of the same caliber.
If you’re choosing ammunition for hunting, read our Short What Ammo Do I Need to Hunt Deer? and our full article How I Choose a Hunting Cartridge: Starting Backward for a deeper look at choosing a cartridge around the bullet and the job you need it to do.
If you’re choosing ammunition for hunting, read our Short What Ammo Do I Need to Hunt Deer? and our full article How I Choose a Hunting Cartridge: Starting Backward for a deeper look at choosing a cartridge around the bullet and the job you need it to do.