Glad to see people are using my whole Prom- naming system, I might become Prom King yet! My first reaction to the Prom Pistol was that it's some kind of flare gun meets facemelter; that is, that fancy burst of fire is some sort of close-range instakill (or otherwise disorienting/incinerating/etc) function but is also headshot-capable. Just an idea on the explanation in an actual firefight:
-Player 1 has Prom Pistol, Player 2 has Assault Rifle.
-Player 2 charges at Player 1 firing.
-Player 1 charges a shot while taking heavy fire; releases it just before Player 2 melees.
-Prom Pistol burst stuns Player 2 with no shields and knocks them back significantly.
-Player 1 lines up the shot and fires a standard uncharged hard light round straight through Player 2's head, vaporizing them.
So, something equivalent to a Plasma Pistol, but with a charged shot that only works in close range and has stun/burst capabilities rather than EMP.