So, my understanding is that the reason the evokers look like guns is to elevate the user’s stress levels and essentially “scare” the persona out of them when they pull the trigger. And obviously, they don’t really fire bullets or the game would be over pretty quickly.
However, do they perform any mechanical function at all? Do they fire some sort of psychic energy, or make a sound, or anything? Or are they essentially just a metal frame with a spring-loaded trigger?
Presumably these questions also apply to Koromaru’s evoker collar, since it doesn’t seem to hurt him.
They have of Plume of Dusk inside them.
Spoilers (final boss, late game party member)
!Plumes of Dusk are fragments of Nyx’s body that were shed when Nyx’s body fused with the moon. They exist in a state between matter and information, having properties of both, and carry within them the same space-time manipulation ability as Shadows - they are essentially physical Shadows. Plumes of Dusk are what allow Mitsuru Kirijo’s motorcycle, Evokers, and the Anti-Shadow Suppression Weapons to function during the Dark Hour, and, in the case of the Anti-Shadow Suppression Weapons, allow them to have a psyche and act as their power source. The Anti-Shadow Suppression Weapons carry within their Personality Module two equally-sized Plumes, crossed in the shape of a butterfly, which is why they are called “Papillon Hearts.”!<
Here’s the page from the Megami Tensei Wiki with that information:
It’s never fully explained, but essentially they’re a machine that forces out your persona though unexplained techno-magic. The shape is just cheesy symbolism and is not integral to the summoning. Helps facilitate stressing the user out in a specific way and puts them in a mind state suitable for summoning a persona.