The logic behind a combination gun is simple: it allows a hunter, who is hunting for food, to be able to take a wide array of game that he might encounter in the field with the appropriate ammo from the same gun.
They look like a double barrel shotgun, but while one barrel is a smoothbore shotgun barrel, the other is a rifle barrel. That is the minimum requirement for a combination gun.
The main idea is to allow a hunter to shoot anything from upland birds to turkey and ducks with the shotgun barrel, and then take medium to large game (depending on caliber) with the rifle barrel, all while utilizing the same firearm. Or, if the rifled barrel is in, say, .22 Long Rifle, it can handle varmints and small game like squirrels without causing too much damage. The decision as to which barrel to use is a simple as choosing one trigger or the other. The rifle/shotguns are quite effective for their purpose.
As such, most combination guns are a break action design and are usually found in an over and under configuration. That brings us to the different types of combination guns.
When the barrels are side-by-side, it is referred to as a cape gun.
If the gun has three barrels (typically two matching gauge shotgun barrels and a rifle barrel), it is known as a drilling, which is German for triplet. However, they were also made with two rifled barrels of different calibers (typically a rimfire and more powerful centerfire cartridge) and one smoothbore barrel, and with with three matching smoothbores or three matching rifled barrels.
Vierlings, which are quite rare, have four barrels. While they were made in a variety of configurations, the most common was two matching smoothbore barrels, a .22-caliber rimfire rifle barrel, and a centerfire rifle barrel--essentially like carrying a double-barrel shotgun and a double rifle at the same time. That's a lot of steel.
Combo guns generally use rimmed rifle cartridges, since they work best with a break-action design.
Combination guns do have their disadvantages. Because of their double barrel design, the point of impact for one of the barrels isn't going to be exactly in concert with fixed iron sights, which is more of a problem for the rifle barrel than the shotgun part. Typically, a combo gun would have a front sight and a rear sight and those would be zeroed to a rifle barrel. It wouldn't be an issue save for very close shots.
Modern combo guns come with scope mounts, which allow for an optic to be sighted to the rifle barrel for reasonable accuracy--but nobody would really want to hunt big game with a combo gun these days if they don't have to.
If you haven't heard of them, that's likely because they were common in Europe, Africa, and Asia, where they were popular with game keepers, but never really caught on in the U.S. A combo gun was used by the Air Force as a survival gun for a time, meant to be a subsistence hunting tool.
The Springfield Armory M6 Scout was a survival carbine built for U.S. aircrews that was originally called the M6 Aircrew Survival Weapon. It came with a .22 Hornet Barrel over a .410 bore barrel. The M6 Scout was identical to the military version and was also made as a .22LR/.410 and a .22 Magnum/.410. Later models have oversized trigger guards for use with gloves.
Combo Guns of Note:
The Savage Model 42 Takedown shotgun-rifle combo is a must-have survival, truck, camping and more gun. This break-action...
The beauty of this combination gun is its takedown ability, which makes it super easy to pack and transport.
This over-under has a free floating cold forged rifle barrel and is made to be extra safe while hunting in the field.
Originally made by Russian-based Baikal and later by Remington, the IZH 94 is an accurate, well-balanced gun that can be found for very reasonable prices in today's used market.
Overall, the idea of a combination gun might seem outrageous at first, but putting together the pieces of the multiple gun puzzle results in a firearm like this. Many manufacturers have taken liberties to create their own versions of combo guns, but the basic tenets still apply.
What sort of use would you get our of a combination gun?