Look at the kill potential of tier 1 mobs, hand cannons will one shot red health tier ones. With the phatom gift perk of the WoC there are a possible 19 kills max. Now with firefly the kill potential for each bullet is 5 (keep in mind the word is potential is not what will realistically happen) I found this number by seeing the radius of the blast and how many mobs would fit in that blast. Now if we multiply 13, the max clip size and multiply it by 5, the kill potential of each bullet,you get 65. with basic maths we can see that 65 > 19. Therefore fatebringer > word of Crota.