Over and under shotgun

A double-barreled shotgun is a shotgun with two parallel barrels, allowing two shots to be fired.

Modern double-barreled shotguns, often known as doubles, are almost universally break open actions, with the barrels tilting up at the rear to expose the breech ends of the barrels for unloading and reloading. Since there is no reciprocating action needed to eject and reload the shells, doubles are more compact than repeating designs such as pump action or lever-action shotguns.

Double-barreled shotguns come in two basic configurations: the side-by-side shotgun (SxS) and the over/under shotgun ("over and under", O/U, etc.), indicating the arrangement of barrels. The original double-barreled guns were nearly all SxS designs, which was a more practical design of muzzle-loading firearms. Early cartridge shotguns also used the SxS action, because they kept the exposed hammers of the earlier muzzle-loading shotguns they evolved from. When hammerless designs started to become common, the O/U design was introduced, and most modern sporting doubles are O/U designs.

One significant advantage that doubles have over single barrel repeating shotguns is the ability to provide access to more than one choke at a time. Some shotgun sports, such as skeet, use crossing targets presented in a narrow range of distance, and only require one level of choke. Others, like sporting clays, give the shooter targets at differing ranges, and targets that might approach or recede from the shooter, and so must be engaged at differing ranges. Having two barrels lets the shooter use a more open choke for near targets, and a tighter choke for distant targets, providing the optimal shot pattern for each distance.

Their disadvantage lies in the fact that the barrels of a double-barreled shotgun, whether O/U or SxS, are not parallel, but slightly angled, so that shots from the barrels converge, usually at "40 yards out". For the SxS configuration, the shotstring continues on its path to the opposite side of the rib after the converging point; for example, the left barrel's discharge travels on the left of the rib till it hits dead center at 40 yards out, after that, the discharge continues on to the right. In the O/U configuration with a parallel rib, both barrels' discharges will keep to the dead center, but the discharge from the "under" barrel will shoot higher than the discharge from the "over" barrel after 40 yards. Thus, double-barreled shotguns are accurate only at practical shotgun ranges, though the range of their ammunition easily exceeds four to six times that distance.

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Wikipedia