What is the final step in correctly matching ammunition to a fire arm?

From the Hunter Education course-

To match the proper ammunition to your rifle, shotgun, or handgun correctly:

• Read the specific caliber or gauge designations on the side of the barrel. Match that designation exactly. For example, if it says “.270 Winchester,” you cannot use “.270 Weatherby.” Shotgun barrels will give the gauge and the length of the chamber (for example, “12 gauge for 2¾-inch shells” or “20-gauge magnum for 3-inch shells”).
• Carefully read the information on the lid of the ammunition box. With shotgun ammunition, always check both the gauge and the shell length, and whether it’s a magnum load, to ensure it matches the data on the barrel.
• Finally, match the information on the barrel to the information on the cartridge or shotshell before you shoot. If in doubt, ask a more experienced shooter or a qualified gunsmith. Some store clerks, although they sell ammunition, may not know about the differences in sizes or the type of firearm you shoot.