Creating custom Ammo Types is not possible at the moment.

There is a planned feature to allowing creation of custom ammo types due to new features of 1.14.

# Ammo type identifier.
ARROW:
  # When 'false' ammo types won't be registered.
  enabled: true
  # Localized name.
  name: Arrow
  # Lore format.
  # Placeholders: 
  # - %name% - Ammo name.
  format: '&7Ammo Type: &f%name%'

SNOWBALL:
  enabled: true
  name: Snowball
  format: '&7Ammo Type: &f%name%'

EGG:
  enabled: true
  name: Egg
  format: '&7Ammo Type: &f%name%'

FIREBALL:
  enabled: true
  name: Fireball
  format: '&7Ammo Type: &f%name%'

WITHER_SKULL:
  enabled: true
  name: Wither Skull
  format: '&7Ammo Type: &f%name%'

SHULKER_BULLET:
  enabled: true
  name: Shulker Bullet
  format: '&7Ammo Type: &f%name%'

LLAMA_SPIT:
  enabled: true
  name: Llama Spit
  format: '&7Ammo Type: &f%name%'

ENDER_PEARL:
  enabled: true
  name: Ender Pearl
  format: '&7Ammo Type: &f%name%'

EXP_POTION:
  enabled: true
  name: Exp Potion
  format: '&7Ammo Type: &f%name%'