D&D 5E (2014) Let's Talk About Guns in 5E

[doctorbadwolf]

If by "martial", you mean Fighter, sure. Everyone else is going to be limited to 2 attacks per turn with a Feat to get around the loading property, and that's if you assume "gun = crossbow" which is super generous and not at all realistic (not that I'm advocating realism here, mind you, but I've already noted what the fire rate of early firearms was like, and it's not pretty).

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.