As opposed to a rifled bore
; refers to a gun barrel
(or the gun which uses it) whose internal hollow (the bore
) is a simple cylinder
rather than having rifling
. The latter, also known as lands
, would be in the form of spirals cut into the inner surface of the barrel.
A smooth bore allows more of the energy of the shot to be applied to the projectile's forward motion. If you're concerned primarily about the energy carried by the projectile (such as with the kinetic penetrators used by modern tank guns) then a smooth bore is preferable. In addition, a smooth bore will not erode as fast as a rifled bore, since it does not present resistance to the hot propellant gases.
In early guns, bores were smooth simply because rifling hadn't been invented yet - and after that because they were easier to manufacture. The primary advantage of the rifled bore - well, see rifling for more information.
Some modern guns are smoothbore. The aforementioned tank guns use smoothbore barrels; stability of the projectile is garnered through the use of aerodynamic fins.