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 left or right side of the barrel. The calibre is also stamped on the cartridge base. The cartridge box indicates the calibre of the ammunition as well. When selecting the correct ammunition to use in your firearm, always consult the data stamp on the rifle’s barrel. It is important not only to match the correct calibre, but also the correct type of ammunition indicated.
For example, cartridges for a .243 Winchester and a .243 WSSM are different and can be dangerous if used in the wrong firearm.
In most cases, the distance between opposite lands inside the barrel gives us the calibre 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 calibre firearm. However, more information is needed than just the bore diameter to purchase the correct ammunition. Always check the data stamp to ensure the ammunition matches the firearm being used.