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 range.
SxS shotguns are often more expensive, and may take more practice to aim effectively than a O/U. The off-center nature of the recoil in a SxS gun may make shooting the body-side barrel slightly more painful by comparison to an OU, single-shot, or pump/lever action shotgun. Gas-operated, and to a lesser extent recoil-operated, designs will recoil less than either. More SxS than O/U guns have traditional 'cast-off' stocks, where the end of the buttstock veers to the right, allowing a right-handed user to point the gun more easily.
The early doubles used two triggers, one for each barrel. These were located front to back inside the trigger guard, the index finger being used to pull either trigger, as having two fingers inside the trigger guard can cause a recoil induced double-discharge. Double trigger designs are typically set up for right-handed users. In double trigger designs, it is often possible to pull both triggers at once, firing both barrels simultaneously, though this is generally not recommended as it doubles the recoil, battering both shooter and shotgun. Discharging both barrels at the same time has long been a hunting trick employed by hunters using 8 gauge "elephant" shotguns, firing the two two-ounce slugs for sheer stopping power at close range.
Later models use a single trigger that alternately fires both barrels, called a single selective trigger or SST. The SST does not allow firing both barrels at once, since the single trigger must be pulled twice in order to fire both barrels. The change from one barrel to the other may be done by a clockwork type system, where a cam alternates between barrels, or by an inertial system where the recoil of firing the first barrel toggles the trigger to the next barrel. A double-barreled shotgun with an inertial trigger works best with full power shotshells; shooting low recoil shotshells often will not reliably toggle the inertial trigger, causing an apparent failure to fire occasionally when attempting to depress the trigger a second time to fire the second barrel. Generally there is a method of selecting the order in which the barrels of an SST shotgun fire; commonly this is done through manipulation of the safety, pushing to one side to select top barrel first and the other side to select bottom barrel first. In the event that an inertial trigger does not toggle to the second barrel when firing low recoil shotshells, manually selecting the order to the second barrel will enable the second barrel to fire when the trigger is depressed again.
One of the advantages of the double, with double triggers or SST, is that a second shot can be taken almost immediately after the first, utilizing different chokes for the two shots. (Assuming, of course, that full power shotshells are fired, at least for a double-barreled shotgun with an inertial type SST, as needed to toggle the inertial trigger.)
Regulation is a term used for multi-barreled firearms that indicates how close to the same point of aim the barrels will shoot. Regulation is very important, because a poorly regulated gun may hit consistently with one barrel, but miss consistently with the other, making the gun nearly useless for anything requiring two shots. Fortunately, the short ranges and spread of shot provide a significant overlap, so a small error in regulation in a double will often be too small to be noticed. Generally the shotguns are regulated to hit the point of aim at a given distance, usually the maximum expected range since that is the range at which a full choke would be used, and where precise regulation matters most.
The double-barreled shotgun is seen as a weapon of prestige and authority in rural parts of India, where it is known as dunali (literally "two pipes"). It is especially common in Bihar, Purvanchal, Uttar Pradesh, Haryana and Punjab.
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- John Barsness (July 2010), "Twin barrel myths: side-by-side vs. over-under", Guns Magazine
- The Popular Dictionary in English and Hindustani and Hindustani and English: With a Number of Useful Tables. Methodist Episcopal Church Press. 1881. p. 48. Retrieved 1 September 2013.
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