I'm a fan of having gunpowder be Dex saves, and I even think the save DC should be set by the gunsmith and not the wielder. And although I understand the historic complaints about armour piercing being exaggerated, there's plenty of D&D weapons that are based on the fantasy of the weapon and not the historical accurate way they were used. IRL you don't use strength to beatdown your opponent with a greatsword, but that's still how D&D does it, and frankly it's right to do it that way because it's still fantasy.
By going with Dex saves, especially a fixed value, it fits the idea that it's a weapon you can equip commoner's with and train them up quickly into an effective fighting unit and not a weapon that is about individual skill of the wielder. However this typically should mean the weapon is basically not meant for the PCs since PCs are all about individual skill. Obviously players can use them, but if the action economy sucks and there's no feats/subclass features that fix it then most players won't use them.
Naturally if the players aren't the ones who are going to use them then what's the point? My answer for this is that it has a very big setting impact, because their power comes from massed volleys. And that means there's a big change to the typical power dynamic. In the past the world needed heroes like the PCs because they were the only ones who could deal with a rampaging dragon. But in a world of firearms, you mass a few hundred militia and that dragon is going down, so who needs adventurers to deal with monsters?
Being part of a dying breed of combatant has a lot real world inspiration to draw from and isn't typical, so should feel fresh. And of course during the campaign the players will show that heroes are still needed even in this new age of science and guns.
Now having said all that if you do want it to be PC wielders then my suggestion would be write the weapon such that when making an attack with a firearm only one no matter how many extra attacks you get, but scale the number of damage dice based on those extra attacks and have an exploding dice mechanic. So for example, if it starts at 2d6, and gains an extra 2d6 for each extra attack and whenever you roll a 6, you add an additional d6 which can also then explode. For feats/class featurs/magic enchantments beyond typical extra damage, or riders that you add, having a focus on dice manipulation like re-rolling 1s &2s, or lowering the exploding die so that it's say 5 & 6 that explode, can bring out the fun of chasing that higher and higher damage totals. Effectively the player is trading multiple low damage attacks for one high damage save, this is typically a bad trade to make so don't be too scared to let the damage really get pumped upwards.
