What characteristic is commonly marked on the barrel or action of modern shotguns?

The characteristic that is commonly marked on the barrel or action of modern shotguns is the gauge. Gauge refers to the size of the shotgun bore and is an important classification that helps determine the type of ammunition the shotgun can use. Shotgun gauges are based on the number of lead balls of a specific diameter that can fit into the barrel, meaning the smaller the gauge number, the larger the bore diameter.

Understanding the gauge is crucial for safe operation and proper ammunition selection, as using the incorrect gauge can lead to malfunctions or dangerous conditions. While other markings such as the model number and serial number may also appear on a shotgun, they do not provide the same critical information regarding the ammunition compatibility that gauge does. Caliber size is a term more commonly associated with rifles and pistols, further distinguishing gauge as the relevant term in the context of shotguns.