No Time To Explain
A single word is etched onto the inside of the weapon's casing: “Soon.”
Novarro's timeline analysis indicates the weapon is the fabled Exo Stranger's Rifle, enhanced at a future point in this continuity and then sent back to this present.
Deliah's timeline analysis indicates the weapon was built by Praedyth, who based it on his own version of the Exo Stranger's Rifle, and then set it adrift in a time ripple.
Hari's timeline analysis indicates the weapon was built by beings of unidentifiable origin, and arrived here by pure accident.
Inachis's timeline analysis indicates the weapon originates from Earth, late Golden Age, and will eventually be lost to time ripples once again, where its systems will degrade and be replaced until our recent past acquires it as the Exo Stranger's Rifle.
As for me... I think it's safe to say the weapon is proving far more fun than we could have hoped.