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Early cartridge shotguns also used the Sx S 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.
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.
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.
Their disadvantage lies in the fact that the barrels of a double-barreled shotgun, whether O/U or Sx S, are not parallel, but slightly angled, so that shots from the barrels converge, usually at "40 yards out".
The original double-barreled guns were nearly all Sx S designs, which was a more practical design of muzzle-loading firearms.
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.
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.
A view of the break-action of a typical side-by-side double-barreled shotgun, with the Anson & Deeley boxlock action open and the extractor visible.The lever and the safety catch can also be clearly seen.