Always Match the Barrel (or Data) Stamp First
All manufacturers use a stamp on their rifles and ammunition. On rifles it is usually located on the left or right side of the barrel and is commonly known as the barrel stamp. The caliber is also stamped on the cartridge base, and is known as a "head stamp". The cartridge box indicates the caliber of the ammunition as well.
In most cases, the distance between opposite landsThe ridges between grooves inside a rifled barrel. inside the barrel gives us the caliber of the rifle or handgunFirearm that is held in the hunter's hands (not braced against the shoulder) when fired.. 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.