Any martial can kill a same-level wizard by acting first, especially if the caster is for some reason relying on cantrips.
Firebolt is garbage compared to any martial using their class features and subclass.
And yeah, if someone is playing a gunslinger they are gonna be using the simple rules in the phb and probably using a bastion facility or simple magic item crafting to have repeating firearms.
Ok. And any same-level Wizard can turn the martial inside out or simply make them unable to do anything. And?
I think we're getting away from the topic a bit. Weapons aren't created with high-level characters in mind. NPC's who are likely to be inventing firearms are going to look at the things in the world around them that they can use. A gun has to be significantly better than other ranged weapons in order to be created in the first place.
After doing a little more research on the topic, both rifles and crossbows are easy to master, and equally able to pierce plate armor at short range. The crossbow was more accurate and had a better rate of fire than early rifles, but technology advanced to the point that firearms became cheaper to produce in mass quantities.
Ultimately, firearms had greater potential than other ranged weapons, but it took time to achieve that potential. Again, I'm not stressing any kind of historical accuracy, but for any discussion about whether or not firearms are balanced in a fantasy TTRPG (such as 5e), we need to select the era of weaponry we're using as our inspiration, and keep in mind the options that already exist in-universe.
I brought up cantrips as an example because the game treats them as efficient, low investment ranged attacks. How easy they are to learn, however, is unclear- for PC's, it's easy to get your hands on something like Firebolt. How true that is for NPC's has to be decided by the GM, since 5e doesn't really give you any indication.
NPC's are rather arbitrarily given abilities. If you took any humanoid NPC and said they were an Elf, suddenly they get a cantrip. But you don't even need to do that, the GM can simply decide "yes, this guy has learned cantrips" if they so choose. Similarly, for PC's, anyone can apparently make a deal with a fiend in a back alley and become a Warlock, gaining Eldritch Blast and upgrades in a short amount of time. Could a fantasy nation field enough Warlocks to make a difference on the battlefield? I have no idea, but if they could, firearms would be woefully inadequate to keep up in a war. Maybe.
After all, the game also gives NPC's access to multiattack at fairly low CR's, long before PC's get Extra Attack. But then again, I've also seen NPC's whose multiattack allows them to make attacks and spellcasting, or even multiple spell attacks, so who knows, really how this would play out.
NPC's might even be able to ignore the Loading property for all I know, if someone decides they ought to.
