In a game where every class has access to damage cantrip on par with crossbow and Wand of Magic Missile is uncommon item, early guns are obsolete. TBH, even crossbows are obsolete. I can't remember someone actually used one in the game. You eather go with archer build or just spam cantrips which have damage scaling with character level. So guns need to do something extra. I would stick with ignoring non magical armor and defenses.
For more steampunkish version, i would just use wand of magic missile and refluff it to 1d6+1 but no auto hit, 7 charges and that's it. Recharge to full after Long rest. Why only 7 charges? Gun has enough steam under pressure for 7 shots, then you need to refill it and steam pump takes time to build up needed amount. Same with rifle, just use d10. With that, you can have it with burst and full auto mode. Burst- 3 charges in one action, full auto- all 7 charges in one action. So, you give player option do lay down suppressive fire, unload on multiple creatures nearby.
