Always Match the Data Stamp First
All manufacturers use a data stamp on their rifles and ammunition. On rifles it is usually located on the side of the barrel. The caliber is also stamped on the cartridge base. The cartridge box indicates the caliber of the ammunition as well.
In most cases, the distance between opposite lands inside the barrel gives us the caliber of the rifle or handgun. In the above illustration, the distance between the lands is 243/1000ths of an inch so this is a .243 caliber firearm. Even though .243 is the caliber of this firearm, there are many different cartridges that use a .243 caliber bullet. For example, .243 Winchester, . 240 Weatherby Magnum, .244 Remington, 6mm Remington International and .243 Winchester Super Short Magnum all use .243 Caliber bullets. It is very important that you match the datastamp on the side of the rifle to the ammunition box and cartridge base to identify the correct cartridge for the rifle. In the above example, this rifle uses the .243 Winchester cartridge. Using any other cartridge would be unsafe.