Combination guns are over/under designs, usually with a rifle barrel over a shotgun barrel. This arrangement allows a wider field of view than a side-by-side or shotgun on top arrangement, since the thinner rifle barrel blocks less of the shooter's vision. Iron sights are used for aiming the rifle, and the front sight alone is sufficient to point the shotgun. One of the more famous combination guns is the Springfield Armory survival gun, sold as models M4 and M6, an all metal folding combination gun in .22 Hornet over .410 bore.
The German and Austrian versions are commonly chambered in 9.3x72R and 16ga, although they were chambered in a wide variety of rifle and shotgun cartridges. They may be encountered in muzzleloading, pinfire, exposed hammer, and hammerless designs.
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 rifle barrel centered on the bottom. A similar arrangement of a side-by-side shotgun with a rifle barrel centered on top, generally a .22 caliber rimfire or .22 Hornet, was also fairly common.
Rarer were the drillings that used two rifle barrels and a single shotgun barrel. These were harder to make, since, like a double rifle, the rifle 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 barrelled shotgun barrels, which are used at shorter ranges with wide patterns of shot where a small misalignment won't be significant. If the rifle barrels were the same caliber, then the three barrels were generally arranged in a triangle, both rifle barrels on top, or one rifle and the shotgun barrel on top (this being known as a cross-eyed drilling). If the rifle 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 barrelled 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. The barrels are all the same gauge.
An unusual but notable drilling is the TP-82, or space gun, is a short barrelled 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 that the unique ammunition it uses had degraded too far to be reliable.
The vierling is almost always in the form of a side by side shotgun, with different caliber rifle barrels stacked vertically, ether both below the shotgun barrels, or one above and one below. Generally the vierling uses one large bore rifle cartridge and one smallbore. Again, the vierling is often custom built to whatever configuration the customer chooses, so any combination is possible. The fünfling is also a custom proposition.
Since these guns tend to be break-open designs, they work best when chambered in rimmed cartridges. Rimfire rounds like the; .22 Hornet, .30-30 Winchester, 7 x 65 R, 8 x 57 IRS and 9.3 x 72 R are all common choices. Gauges tend to be large, 16 or 12 being most common. Since drillings and vierlings are primarily European, American calibers are rarer and, at least in the American market, more desirable and expensive. 20 gauge drillings and vierlings also command a premium due to the relative rarity compared to the larger gauges. Many pre World War II European guns are chambered for 2 9/16" shotgun shells, and will need to be rechambered to fire modern 2 3/4" shells.
Combination guns show a fairly wide range of calibers and gauges; Savage Arms, for example, made models from .22 LR over .410 bore shotgun up to .308 Winchester over 3" 12 gauge magnum. The Springfield Armory survival guns are typically .22 Hornet over .410 bore.
It is not uncommon to find combination guns paired with sub-caliber inserts for the shotgun barrels. These inserts, usually 9 to 11 inches in length, slip inside the shotgun barrel(s) and chamber rimfire cartridges such as .22 LR or .22 Magnum. These further increase the flexibility of the guns, and will add considerably to the value.