It's weak and useless at the moment, so let's make it even to the Callahan or maybe even Valiance.
What could work:
A - Move stun from Callahan to Iron Slug and increase Callahan's damage and interrupt chances
B - Keep stun on Callahan and add it to Iron Slug too, debuffing Callahan a bit
C - Increase the combo to be 3-5 instead of 2
D - Increase the damage
E - Let the user move while firing
F - Inrease ASI for firing AND bullets
Stun is unique to the Callahan, so A and B might not work best but it's just a theory.
I say the options must be mixed and experimented with for best effect.
I don't want E. The stopping for firing makes the gun feel like it packs a punch, which is more important than it actually packing a punch.
D is nice. The more damage Iron Slug and Callahan gets, the better Iron will be compared to Calla since weak-neutral stays the same but neutral-strong will widen.
Unless they plan to fix internal ricochets, Iron Slug should do at least the damage of two Nova Driver shots per shot.