When a shotgun slug is rifled, it means that the slug itself has spiral grooves cut into its surface. These grooves, known as rifling, are similar to those found in the barrels of rifled firearms such as rifles and handguns. The purpose of rifling in a shotgun slug is to impart spin to the slug as it travels through the barrel.

The spinning motion created by the rifling stabilizes the slug in flight, similar to how it stabilizes a bullet fired from a rifled barrel. This stabilization helps improve accuracy and enhances the slug's ability to maintain a stable trajectory, particularly over longer distances.

Rifled shotgun slugs are typically used in smoothbore shotguns, which have barrels without rifling. By using a rifled slug, shooters can benefit from improved accuracy compared to shooting traditional non-rifled slugs through a smoothbore barrel. However, it's important to note that rifled slugs are designed to be used with smoothbore barrels, not with rifled shotgun barrels.

It's worth mentioning that there are also specialized sabot slugs available for shotguns with rifled barrels. These slugs are encased in a plastic sabot, which engages with the rifling in the barrel, allowing for increased accuracy and range.