[...Ah. This one is small. Hopefully it's the younger self of the one he usually talks to, who awakened to his Persona in Inaba, because otherwise that means there are two thirteen-year-old Akechis wandering the Metaverse, and that's not ideal.]

If you mean a gun in general... Well, putting aside that I probably shouldn't be telling children where to obtain illegal firearms, I usually pick them up from black marketeers.

If you're looking for one that works as this one does, however -

[One more Shadow, in the distance. He sights, he fires - a bullet too large for the actual barrel of the gun shoots out in a streak of blue-green before exploding into ball lightning at the end.]

I'm afraid it's nothing to do with the weapon at all. It's simply my preferred tool for focusing psychs.