Which of the following is a defining feature of a shotgun?

A defining feature of a shotgun is its barrel length. Specifically, in many jurisdictions, including Scotland, a shotgun must have a barrel length of not less than 24 inches. This requirement is significant because it delineates shotguns from other types of firearms, particularly handguns and short-barreled rifles that have different licensing and regulatory requirements. The 24-inch minimum barrel length contributes to the classification of the weapon as a shotgun, which is designed for firing multiple small pellets or shots, making them distinct from other firearms with different characteristics.

Other options do not accurately reflect proper definitions or regulations concerning shotguns. For instance, identifying a shotgun as "not a firearm" contradicts the classification since shotguns are indeed categorized as firearms. The stipulation about magazine capacity is more relevant to other firearm types and not specifically defining for shotguns, which can have varying magazine sizes. Additionally, the description of shotguns as "must be an air weapon" is inaccurate; air weapons are a separate category altogether and do not fall under the traditional definitions of shotguns, which are typically firearms that use gunpowder to propel projectiles.