(Not sure how balanced they would be against other ranged stun weapons, though...)
Make it a researchable clip that uses Elerium Batteries to produce greater electric shocks with more power and less weight, translating into a special magazine that requires Elerium to build that does more damage, can shoot 4 times, has better range and weights less.
In terms of effectiveness it would be behind the Stun Launcher, the ultimate non-lethal weapon. There are better options in the Power Mace and Stun Rods, but these require melee, which makes the improved Heavy Taser better until the endgame.
The dart guns are okay too, but they take ages to build and don't work against zombies and machines, so it should be okay.