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 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. Knowing this, we can purchase the correct ammunition for a firearm.