The Shotgun in Sons of the Forest can be loaded with Buckshot Shells for close-range attacks or Slug Ammo for medium-range attacks. If you want to change ammo types, look towards the ground with the Shotgun equipped and select the prompt. Note that this will unload your Shotgun and load the other kind of ammo, meaning you will be defenseless for some time, but you can fire the Shotgun without fully loading it.