A combination gun is a firearm that comprises at least one rifled barrel and one smoothbore barrel, that is typically used with shot or some types of shotgun slug. Most have been break-action guns, although there have been other designs as well. Combination guns using one rifled and one smoothbore barrel usually are in an over and under configuration. Side-by-side versions are referred to as cape guns. A drilling (German for "triplet") is a combination gun that has three barrels. A vierling (German for "quadruplet") has four barrels. Combination guns generally use rimmed cartridges, as rimless cartridges are more difficult to extract from a break-action weapon.
Combination guns have a long history in Europe, Africa, and Asia, dating back to the early days of cartridge firearms. These guns are almost exclusively hunting arms. The advantage of having a single firearm that can fire both cartridges designed for rifled and smoothbore barrels, is that a single gun can be used to hunt a very wide variety of game, from deer to game birds, and the shooter can choose the barrel appropriate for the target in seconds. As a result, they are popular with gamekeepers who often need the flexibility of the combination gun during their activities.
The earliest combination guns were called swivel guns (not to be confused with the more widely known small cannon), which used a set of barrels designed to rotate to allow either the rifled or smoothbore barrel to line up with a flintlock mechanism. Modern combination guns tend to resemble double-barreled shotguns and double rifles, and are almost universally break open designs. Combination guns generally have a selector that allows the user to choose which barrel will fire. Drillings with two shotgun barrels and one rifle barrel may have two triggers, one for each shotgun barrel, and a selector that will allow one trigger to fire the rifle barrel. Four-barrel versions known as Vierlings generally have two triggers, and selectors to switch each between shotgun and rifle.
Combination guns are over/under designs such as the Savage Model 24, usually with a rifled barrel over a smoothbore barrel. Iron sights are commonly used for aiming the rifle, and the front sight alone is sufficient to point the shotgun. Scope mounts are available for some designs.
An interesting combination gun is the United States Air Force M6 Aircrew Survival Weapon and its civilian version Springfield Armory M6 Scout, an all-metal folding combination gun in .22 Hornet over .410 bore or .22 Long Rifle over .410 bore.
A cape gun is a side-by-side version of a combination gun, and is typically European in origin. These were at one time popular in southern Africa where a wide variety of game could be encountered. British versions are commonly chambered for the .303 British service cartridge and a 12-gauge smoothbore barrel, with the rifled barrel positioned on the left. German and Austrian cape guns have the rifled barrel on the right side, which is fired by the front trigger. The front trigger is usually a set trigger as well. The German and Austrian versions are commonly chambered in 9.3×72mmR and 16-gauge, as were commonly carried by the old gamekeepers, although they were chambered in a wide variety of rifle and shotgun cartridges.
Drillings (Drilling: German for triplet) normally consists of two matching smoothbore barrels and a rifled barrel (German: Normaldrilling, common drilling), but may cover a much broader range of shapes and configurations:
Since drillings were generally made by small manufacturers, each maker would pick whichever layout they preferred, or whatever layout the customer ordered. The most common layout was a side-by-side shotgun with a centerfire rifled barrel centered on the bottom, such as the M30 Luftwaffe drilling. A similar arrangement of a side-by-side shotgun with a rifled barrel centered on top, generally a .22 caliber rimfire or .22 Hornet, was also fairly common.
Rarer were the drillings that used two rifled barrels and a single smoothbore barrel. These were harder to make, since, like a double rifle, the rifled barrels must be very carefully regulated, that is, aligned during manufacture to shoot to the same point of aim at a given distance. This requires more precision than regulation of double-barreled shotgun barrels, which are used at shorter ranges with wide patterns of shot where a small misalignment won't be significant. If the rifled barrels were the same caliber, then the three barrels were generally arranged in a triangle, both rifled barrels on top, or one rifled and the smoothbore barrel on top (this being known as a cross-eyed drilling). If the rifled barrels differed in caliber, generally the layout would be an over/under using the shotgun and a centerfire rifle barrel, with a rimfire rifle barrel mounted between and to one side. These configuration, with shotgun/centerfire/rimfire barrels, are the most desirable configuration for modern collectors.
The triple barrel shotgun is the rarest configuration, and arguably is an odd variant of a double-barreled shotgun rather than a drilling, since it lacks the rifle/shotgun combination that all the other drillings have. The triple barrel shotgun is generally laid out like a side-by-side shotgun, with the third barrel centered and below the other two. Although, the Chiappa Triple Crown has a triangular arrangement with one on top and two below. This gives the Triple Crown a single barrel sight picture. The barrels are all the same gauge.
An unusual but notable drilling is the TP-82 Cosmonaut survival pistol, a short-barreled drilling pistol consisting of two 12.5 mm smoothbore barrels over a 5.45 mm rifled barrel, with a detachable shoulder stock. It was developed by the Soviet Union as a survival gun for their space program, and was in use from 1987 to 2007, when it was retired due to the fact that the unique ammunition it uses had degraded too far to be reliable.
Vierlings ("vierling" being German for "quadruplet") generally consist of two matching smoothbore barrels, a .22 caliber rimfire rifled barrel and a centerfire rifled barrel. Although, they can come in a variety of configurations. Vierlings are quite rare and are almost always custom made for the high-end commercial market. One example was the four barreled Lancaster carbine, originally designed for the Maharajah of Rewa for hunting tigers.