TheHunter: Call of the Wild Wiki

"Put your prey to bed with these powerful slugs for any 16-gauge shotgun.

A slug is a shell with one massive projectile encased instead of several smaller ones. This gives it greater effective range compared to multi-projectile shells such as buck and birdshots, and allows it to be used against bigger game.
― In-Game Description

The 16 GA Slug is an ammunition type available for the Grelck Drilling Rifle family of shotguns.

This ammo is available as part of the Weapon Pack 2 DLC.