TheHunter: Call of the Wild Wiki

"These 16-gauge shells are perfectly balanced for speed and pressure to make sure that you get the most out of your shotgun.

Bird shot contains a large amount of small pellets. This gives it short effective range and low damage. However, the wide and dense pattern generated by these properties also makes it ideal for hitting small fast-moving targets such as small game and birds.
― In-Game Description

The 16 GA Birdshot 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.