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There's no particular reason, fluff-wise, why D&D style firearms have to be particularly worse than D&D style crossbows, or other missile weapons. Of course, missile weapons back in 3.5 were terrible to begin with, so there you go.
First thing you could do is re-fluff it slightly so there's more of an alchemical bent to the weapons/gunpowder. After all, this is a setting where you can find magical weapons lying around at level 1-2 anyway; no reason every firearm in the world is historically-appropriate-shitty. Maybe you have paper cartridges and a sort of breech-loading mechanism, so it's slighly less bullshit that you can be reloading it every round. Multiple shot attacks become double charges, or shotgun pellets, or what have you. Adding flammables or acid capsules to your loads gives you elemental damage. That kind of thing.
Ultimately, you could have weird things like a magic blunderbuss that eats scrap metal and converts it to bullets, or that sort of thing.
Alternatively, you could build it sort of like the way the Witch Hunters played in Warhammer Online. You have a melee weapon and a ranged weapon, and you tend to save the ranged weapon for close-in or opportunistic shots. So something like, you stab-stab-stab, and then step up to point blank and pull the trigger in his face. But by that point, you're basically inventing a whole new class, including attack abilities.