I'm probably not the first to note this, but at the end of Ciel's arc, more or less right after you've made the final choice that triggers the so-called "Good Ending," Ciel returns from the Vatican, and Shiki notes that her robes are literally bulging, "fully equipped for war." Arcueid says to her something like "I see you have the Black Barrel; I guess the rumors about you trading useless information to Enhance were true." According to the dialogue, Ciel intends to use the Black Barrel to seal Arcueid. Now I was under the impression that while Sion was in possession of the replica, the true Black Barrel was itself sealed away with the other terrible conceptual weapons developed at ATLAS (and that it supposedly would play a role in the distant future when it was discovered in the excavated ruins of the installation).
Is this conversation implying that Enhance somehow came into possession of it during this time period? I know Enhance is the Dead Apostle ancestor who is intent on eliminating all of the other dead apostles, and for this reason has cooperated with both the Church and Arcueid/Shiki. But I don't see how he could have gotten that weapon from ATLAS, since it was one of those doomsday weapons that they were constantly inventing in hopeless pursuit of preventing a foreseen apocalypse, and then locking away in a rather similar manner to the US's nuclear arsenal. Even a mere replica carried by Sion was a devastating conceptual weapon (in canon and not necessarily the fighting portion of MB, where people kept coming back for more after being nailed by the 7th holy scripture or Shiki's death strike), although not enough to take care of Arcueid.