Final answer:
The gauge of a shotgun is marked on the rear of the barrel, and the gauge of a shell is marked on the shell casing. It is critical to use the correct size shell for the shotgun gauge to ensure safe operation.
Step-by-step explanation:
The gauge of a shotgun is usually marked on the rear of the barrel, and the gauge of a shell is marked on the shell casing. The gauge number is an indication of the shotgun's bore size and corresponds to the size of the shotgun shell that should be used with it. It is important to match the gauge of the shell with the gauge of the shotgun for safe and proper operation. Different gauges indicate the diameter of the barrel and hence the size of the shell required; popular gauges include 12-gauge, 20-gauge, and .410 bore among others. Markings on both the gun and the shells help ensure that only the appropriate ammunition is used with a particular firearm.