What I Tested:
For Moze, it appears that bullet multiplication mechanic only applies to the four Call guns (Rowan’s, Lucian’s, King’s and Queen’s), the Recursion, and the Carrier. It does not apply to any other Jakob’s gun (legendary or non-legendary) or gun that shoots bullets that pass through a target and/or reflect of walls (the Ghaash, Juliet’s Dazzle, etc). I have not tested the Redistributor or Brainstormer yet, though I suspect their mechanics work differently as well.
What is Bullet Multiplication and why do it?
Simply put, using elemental damage bonuses to cause extra bullets to spawn whenever you hit an enemy. These extra bullets will reflect around the environment and do bonus damage to your target (in the case of the King’s and Queen’s Call), other enemies (Rowan’s and Lucian’s Call), or both (Recursion and Carrier). This is essentially free DPS and who doesn’t like free stuff, right?
Well, how you do you it?
By adding bonus elemental damage to your bullets. For Moze, elemental damage can come from Moze’s skills (Experimental Munitions and Cloud of Lead), the Green Monster class mod, and weapon, grenade and shield anointments. The incendiary damage from Stoke the Embers and elemental damage rolls that you can get on artifacts are only additive to the element damage you are already doing, so they do not spawn extra bullets.
This comes with the caveat that some elemental damage bonuses can stack on top of each other if they are of the same type and each time they do, it will spawn one fewer bullet.
So what stacks?
The Green Monster stacks with nothing.
Moze’s incendiary damage skills stack with incendiary damage anoints on weapons.
Elemental damage anoints on shields and grenades stack.
So the theoretical max number of extra spawns you can have going on your character at one time is five – one from the Green Monster, one from your skills, one from your weapon anoint, one from your shield, and one from your grenade. The gun’s actual elemental damage type does not effect stacking.
One final note:
We’re not done yet! The last factor in all of this is your weapon. The Carrier will only spawn one extra bullet per instance of bonus elemental damage, but as it says on their item cards, the Rowan’s and Lucian’s Call will spawn 2 extra bullets per instance and the King’s and Queen’s Call will spawn three.
As for the Recursion, I’m still trying figure that out. Suffice to say, it causes the projectiles to go crazy and do a lot of damage
Here is an example of a Rowan’s Call with all 5 spawning opportunities, creating 10 ricochets in total.