#Either Lucian or Senna are Durand's descendant, and their guns are made out of Petricite.
We know that Lucian (and supposedly Senna) come from Demacia.
Durand, Galio's builder, wrote a book about Petricite's properties, one of which being that it stocks the magic it absorb and that it can be released, a property seemingly unknown by anyone but him considering Sylas discovers it reading the book. Unknown by anyone, except maybe his lineage.
From generation to generation, the secret was kept and eventually Lucian or Senna learned of it as well.
They were then able to obtain the gun and go on their rampage against corruption and everything.
But then comes another theory regarding the guns or, more precisely, Petricite:
**What if Petricite also had some filtering properties?**
If the guns are made out of Petricite, eventually the magic they contain would run out at some point, meaning they are "recharged" by something, and one of the main source of magic that could replenish them would be the Shadow Isle's magic.
However, the magic coming from the guns don't look very *shadowy*, but let's keep in mind that the Shadow Isle's magic is corrupted Blessed Isle's magic.
The guns (and in consequence, possibly Petricite) might hide a yet undiscovered property allowing them not only to absorb magic but also to filter it or restore it to its original state, allowing Lucian and Senna to use the Blessed Isle's magic through it.