A combination gun is a firearm that usually comprises at least one rifled barrel and one smoothbore barrel, that is typically used with shot or some type 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 are commonly found in an over-and-under configuration, while the side-by-side configuration is usually referred to as a cape gun.
A combination gun with more than two barrels is called a drilling with three barrels, a vierling with four barrels, and a fünfling with five barrels. Combination guns generally use rimmed cartridges, as rimless cartridges are usually more difficult to extract from a break-action firearm.
M6 Aircrew Survival Weapon
Common drilling barrel arrangement side-by-side shotgun barrels over a rifle barrel
Scoped drilling with a shotgun, centerfire rifle and rimfire rifle barrels.
Break action is a type of firearm action in which the barrel(s) are hinged much like a door and rotate perpendicularly to the bore axis to expose the breech and allow loading and unloading of cartridges. A separate operation may be required for the cocking of a hammer to fire the new round. There are many types of break-action firearms; break actions are universal in double-barreled shotguns, double-barreled rifles, combination guns, and are commonly found in single shot pistols, rifles, shotguns, including flare guns, grenade launchers, air guns, and some older revolver designs. They are also known as hinge-action, break-open, break-barrel, break-top, or, on old revolvers, top-break actions.
A view of the break-action of a typical double-barrelled shotgun, with the action open and the extractor visible. The opening lever and the safety catch can also be clearly seen.
Close-up of an IOF 32 break-action revolver
Smith & Wesson Model 1 Third Issue open