Chuck,
A
slug barrel is a barrel for a
shotgun that is designed primarily to shoot
slugs.
Until widespread use of the
sabot slug starting in about 1985, most slug barrels were
unrifled, having a smooth bore. They differed from standard shotgun barrels by the absence of a
choke.
When slugs are fired in a standard, choked barrel, the slug is deformed by the choke as it exits. The degree of deformation is most acute with fuller chokes, which were among the most widely used in stock shotguns up until about 1990.
Sabot slugs offer an accuracy benefit only when fired from a rifled barrel, because the slug will tumble end for end without the spin imparted by the rifling.