Combination guns are (usually) long guns chambered for two different calibers. For example, one can purchase a gun chambered with a .410 gauge shotgun barrel on top and a .22 LR rifle barrel on the bottom. Such guns allow a hunter to shoot a variety of animals, or the same prey in different situations.
There are also hunting rifles called "drillings" which feature three barrels, each with a different caliber. James R. Rummel has a post about these unique firearms. He proposes getting one a .22 LR barrel for small game, a .357 magnum for deer hunting, and .375 magnum for large, dangerous game.
http://hellinahandbasket.net/?p=4981 | Photo: Centerline Firearms