A long gun is a firearm with an extended barrel, usually designed to be fired braced against the shoulder. Barrels of such weapons commonly extend to around 50 cm or longer, giving considerable accuracy and range. Most modern long guns fall into one of two categories, rifles or shotguns, distinguished by their design and type of projectile they fire. Historical examples of long guns include muskets, blunderbusses and wall guns.

Long guns are at the opposite end of the firearm size spectrum from derringers and handguns, and are longer than carbines.

In historical navy usage, a long gun was the standard type of cannon mounted by a sailing vessel, called such to distinguish it from the much shorter carronades. In informal usage, the length was combined with the weight of shot, yielding terms like "long 9s".