Like all La Vaule (S) NM Level 69 Weapons, you'll need the Supremacy Earring to get the full "Set:" effect. The earring drops from the BCNM that is granted access if you defeat all 8 NMs in La Vaule and trade the 100% StrongBox items in to a NPC Named Lurouillat in Southern San d'Oria (S).
Those items also benefit San d'Oria in Campaign. Look up the quest called "Sealed Containers."
Ever since i started doing nyzul and finally getting 100 floor and unlocking Trueflight ive been looking to get one of these and the earring to make the SET.
So any rng who has Trueflight Weaponskill will love this gun if they are able to make the SET since MAB+5 = +5% Damage with Trueflight.
Until you find out what the rest of the "set" consists of, Coffinmaker, the current COR favorite is still slightly better. Although 'Murderer' sounds cooler.