What tactically interesting play there is in 5e sort of falls apart with ranged weapons. Moving at half-speed lets you be totally effective with the weapon. Cover is negated by a number of cheap mechanics. Damage is high enough compared to firing rate and movement even without firearms that enemies that would utterly destroy you at close range don't even survive to hit you once if you can engage anywhere close to max range.
Even without firearms, the 5e ranged weapon rules already render melee combat pretty close to obsolete. Only the convention of small cramped combat areas cause it to not utterly dominate. Every melee combatant needs to invest in insane mobility (or stealth) and use it to close the gap on ranged foes; ranged combatants only need to expend a bit of mobility or stealth after the melee foe both closes the distance and isn't defeated first.
Having firearms whose damage output is significantly larger just increases this gap, and no appeal to realism really works. You have to appeal to what kind of fights you want really, or provide some other huge advantage to melee, to keep it viable. All fights happening in twisty corridors, lots of darkness (magical or not) preventing use of ranged weapons, etc.
And ignoring the massive tactical advantages, the mini game of ranged combat in 5e is more boring than that of melee. Melee cares more about positioning, and at some levels opportunity attacks let you defend other characters by engaging foes before they focus fire. Ranged combat does away with cover pretty fast, and then positioning (other than full cover) matters very little. This in turn makes focus fire the obvious effective strategy.
Now, 5e melee combat mini game isn't the most engaging; it falls short of 4es melee combat mini game (heck, it falls short of 4e's ranged combat mini game!). But it has at least something going for it.
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All of that grousing being said, I'm trying to introduce magical firearms in a campaign I'm working on.
Instead of being slug throwers, they are magical in nature. I have a gameplay loop I want, so the weapon mechanics are based on that gameplay loop.
The basic one is a lighting throwing weapon. They deal impressive damage, but they use the wielders "soul" or "mana" as a battery. This gives them recharge mechanics that you can't bypass by simply wielding more of them.
There are 3 personal shockguns - the pistol, the rifle and the boomstick.
The sparkpistol is intended to be used as an off-hand weapon. You fire it, then let it recharge while fighting with the other weapon. Swashbuckling is the feel I'm going for.
The sparkrifle is two handed weapon. A soldier would shoot, then take cover and recharge, and repeat. It can have a bayonette attached turning it into a spear for close quarters fighting.
The boomstick is a short ranged two handed aoe weapon that recharges slowly. Great for closing in, using, then switching to melee weapons. It is still called a sparkgun in-world, because thundergun is restricted to siege/ship weapons.
Ship thundercannon fire a lighting "tracer" attack relatively rapidly, then once they hit a target follow it up with a thunder-based explosion.
I've got reasonable mechanics for these. I'm working on some mechanics for the "opposing faction", magma-throwing weapons, but haven't got ones I'm happy with yet.
Maybe this is copping out. But because I'm inventing the rules of these magitech weapons, I can modify them to get the gameplay loop I want, instead of having to deal with "what is realistic" for "standard" guns.
