I bought a TFWPKY 1969 thinking that the 331 attack and the slight impact buff would make more damage than the Fatebringer, but it doesn't. The glorious Fatebringer is still making more damage (due to the explosive rounds).
I'll add some data soon.
I've tested them through the 2nd, 3rd and last parts of the CE raid.
Please, feel free to add some relevant info/data.
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The thing about damage output and CrE is that a thrall is a 1 headshot kill (with any weapon) and with firefly you're able to take out all his nearby buddies. So FB is very useful in CrE, it's perfect for thrall mobs.