An under/over firearm, most commonly a shotgun, is a type of firearm with two barrels arranged vertically, one stacked on top of the other. This configuration offers a few key advantages:  

Single sighting plane: The over/under design places the barrels in a single vertical line, which some shooters find provides a more natural and intuitive sight picture compared to side-by-side shotguns. This can contribute to improved accuracy.  

Improved swing: The slimmer profile of an over/under shotgun can make it easier to swing and track moving targets, which is beneficial for hunting and clay shooting sports.

Aesthetics: Many shooters appreciate the classic and elegant appearance of over/under shotguns.

Choke versatility (sometimes): Some over/under shotguns allow for independent selection of chokes for each barrel, providing flexibility for different shooting scenarios. For example, you might use a more open choke in the bottom barrel for close-range shots and a tighter choke in the top barrel for longer-range shots.

Over/under shotguns are commonly used for:

Hunting: They are popular for upland game hunting, waterfowl hunting, and some big game hunting (depending on the caliber).

Sporting clays, skeet, and trap: These clay target shooting disciplines often favor over/under shotguns due to their handling characteristics.  

Recreational shooting: Many shooters simply enjoy the feel and balance of an over/under shotgun.

While over/under shotguns are popular, they also have some potential drawbacks:

Capacity: They typically hold only two rounds, requiring more frequent reloading compared to some other types of shotguns.

Cost: Over/under shotguns can sometimes be more expensive than other types of shotguns, particularly those with high-quality craftsmanship and features.

In summary, the over/under shotgun is a classic and versatile firearm that is well-suited for a variety of shooting activities. Its single sighting plane, smooth swing, and aesthetic appeal make it a favorite among many shooters.