Bullets are going to be as-lethal as the poster writes they are. We don't know Merlwyb's need for testing the armor, or what caliber shot she used, if such a thing even exists in Eorzea. It won't take much more than someone else saying "These are specifically made to shoot through armor" to have that be handwaved away anyway. Such things exist in real life, after all. We have slugs that shoot through tank armor, and Eorzea's a world where magic exists.
That said, I imagine they're not simple to obtain (for now). They're reasonably expensive to maintain, ammunition would be costlier than a sword or a lance for the common man and while there's certainly evidence that guns are used by certain official offices, I don't think it's unreasonable for the average citizen to have a hard time getting their hands on one.
Of course, adventurers aren't common citizens, and we have RP shells where folks are building entire airships with combustible engines and power armors and magiteck mounts, so a firearm should be simple in comparison.
That said, I imagine they're not simple to obtain (for now). They're reasonably expensive to maintain, ammunition would be costlier than a sword or a lance for the common man and while there's certainly evidence that guns are used by certain official offices, I don't think it's unreasonable for the average citizen to have a hard time getting their hands on one.
Of course, adventurers aren't common citizens, and we have RP shells where folks are building entire airships with combustible engines and power armors and magiteck mounts, so a firearm should be simple in comparison.