(n.) A gun barrel which is not rifled. Shotguns are smoothbored. So is the 120mm cannon on the M1A1 Abrams tank.

of a firearm; not having rifling or internal spiral grooves inside the barrel

A rifle with an unrifled bore, such as musket, blunderbuss etc. Also sometimes used to describe a shotgun.

Firearm with unrifled bore.

A gun which is not rifled and whose barrel is completely 'smooth' all the way from breech to muzzle. The projectile is not spin-stabilised and hence the guns are relatively inaccurate. The most common modern gun that is a smoothbore is a shotgun.

A gun is smoothbore if the inside of the barrel is completely smooth. Smoothbore guns were used before rifled guns were developed. Although smoothbores were not as accurate and had a shorter range than rifled arms, there were still plenty of them in use during the Civil War.

Smoothbore refers to a firearm or cannon which does not have a rifled barrel.