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 from which they evolved. 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.
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 O/U, 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 slightly to the right, allowing a right-handed user to point the gun more easily.
Double shotguns are also inherently more safe, as whether the shotgun is loaded or can be fired can be ascertained by anyone present if the action is broken open, for instance on a skeet, trap or hunting clays course when another shooter is firing; if the action is open, the gun cannot fire. Similarly, doubles are more easily examined to see if loaded than pump or semi-automatic shotguns, whose bolt must be opened and chamber closely examined or felt to make sure it is unloaded; with a double gun (or a break-action single gun), whether the gun is loaded, i.e., has cartridges in any chamber, is easily and immediately seen with a glance (and just as easily unloaded).
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 very undesirable 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 shotgun and shooter, particularly if it was unanticipated or unintended. 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 (this also can happen if the first shell fails to fire). 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, without removing the gun from the firing position on the shoulder and without any other action than a second trigger pull, 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.) This can be noticeably faster than a pump shotgun, which requires pumping to eject and reload for the second shot, and may be faster, or not slower, than a semi-automatic action. Note, however, in neither the pump or semi-automatic will the second shot be a different choke pattern from the first shot, whereas for a double, the two shots are usually with different chokes. Thus, depending on the nature of the hunt, the appropriate choke for the shot is always at hand. For example, while field hunting flushing birds, the first shot is usually closer than the second because the bird flies away from the shooter; so, the more open choke (and barrel) would be better for the first shot, and if a second shot is needed, as the bird is flying away, the more closed (and thus longer distance of an effective shot pattern) choke (and barrel) is then appropriate. Conversely, on a driven hunt, where the birds are driven towards the shooter, the closed (longer effective distance) choke (and barrel) should be fired first, saving the open (closer effective distance) choke (and barrel) for the now-closer incoming bird. None of this is possible with single-barrel shotguns, only with a double, whether SxS or O/U.