A blunderbuss is a muzzle-loading firearm with a flared, trumpet-like barrel and is the predecessor to the shotgun. Most of these weapons are mid-sized, being smaller than most shoulder-fired arms, but larger than a pistol. Although fitted with a butt, the dimensions suggest that most were not really intended to be fired from the shoulder and were instead fired from the hip. The compact dimensions of a blunderbuss would facilitate use in small spaces (e.g. on a ship, or in a house) and would also make storage easier. For those requiring an even smaller weapon, blunderbuss pistols were also produced.

The funnel-shaped barrel (either round or elliptical) is not designed to enhance the ballistics of the weapon, but serves to facilitate loading ammunition into the muzzle. This makes it much easier to refill a blunderbuss with shot in situations where this would not normally be possible (as when riding shotgun on a stagecoach speeding down a bumpy road or on the decks & riggings of a ship).

While there is no physical reason that a blunderbuss cannot fire projectiles such as gravel or nails instead of lead shot (as is often claimed) this would be a foolhardy action as it would result in the barrel's ruin. Blunderbusses were often supplied with gang-moulds by their manufacturers, allowing the user to make his own shot in the field. A blunderbuss can fire multiple balls simultaneously, and generally discharges its entire load at once. This made the blunderbuss the ideal weapon for boarding ships.