TheHunter: Call of the Wild Wiki

"Versatile and powerful, this slug gives the 20-gauge shotgun enough stopping power to take down even medium-sized targets with a single shot.

A shotgun slug is a shell with one massive projectile instead of several small ones. This gives it greater effective range and allows it to be used against bigger game than multi-projectile shells such as buck and bird shot.
― In-Game Description

The 20 GA Slug is an ammunition type available for the Strecker SxS 20G and Nordin 20SA families of shotguns.

This ammo is available as part of the Wild Goose Chase Gear and Duck and Cover Pack DLCs.