First off you have to know what a derringer is. In general a derringer is a small, powerful, single shot gun (recent advances in gunsmithing has allowed there to be 2 shot derringers but the first ones were single shot guns)
Currently in the game the closest thing we have to a Derringer is the Magnus lines.
This handgun I am suggesting is more powerful than any magnus line and can deal as much damage in one hit as a BAB (as a 5star gun and only on one enemy mind you). It comes with a hefty flinch rate, bullet speed and damage, but is balanced by being normal damage, firing mechanics comparable to magnus lines (can't move while firing), and having a single bullet clip (meaning you must reload).
The Charge attack fires a large bullet like the magnus lines, wherever the bullet lands it will deal damage and then create a small vortex (3star bomb equivalent) that slowly moves towards where the charge was fired.
Without the charge attack few people would use this gun and it would become something like a Blitz needle but instead of damage it would be incredible utility (imagine pulling enemies into an ally's haze bomb, or setting up for an ally's brandish charge, or just grouping up shocked enemies for that damage spread)
Utility-wise, it's unique. However, weapon-wise, it doesn't work. In order to charge a weapon, you must attack first. With a single-shot clip, this requires players to shoot, reload, charge - which may be good for balance sake if you want to think of it that way. This is ignoring techniques such as shield-charging and whatnot, but that is a tactic, not the method.
An idea to throw into the mix is having the charged bullet's explosion be a small, quick-lived vortex bomb, and have its charge shot pierce through enemies. Enemies near the bullet's end position will be pulled towards it, it will appear a set amount away (unless it hits walls), so it becomes a gunner's utility rather than a vortex replacement.
Having a bullet that pulls enemies along with it seems like a great utility, but we already do that, and with less flinging involved(Brandish charges).