When you make a new ranged weapon, you can choose a bullet for it, only one bullet.
If you want to make a ranged weapon with multiple bullets or a new arrow for vanilla bow you can't.
My suggestion is that there is an option for items "is item a bullet" that, if enabled, will unlock a few options for that item:
Damage of the bullet
Texture/3d model of shot bullet
New procedure triggers for that bullet item:
When bullet hits block, when bullet hits entity, etc.
And an option for choosing if the bullet is compatible with vanilla bow.
You can choose multiple bullets per gun. You can still choose non-bullet items, and they'll use the texture/3d model chosen in the ranged item editor. If you choose a bullet item, it will instead use its own texture and 3d model. The procedure triggered by the ranged item and the bullet will merge.
If you choose a bullet item for a ranged mob, it will use its texture/custom 3d model.
Bullet items can also be fired by dispensers.
I know that this suggestion is quite big and won't be added soon, but I hope that you will consider it.