They aren't dependent on readily available magical healing, this is just the thread diverging on the tangent of "why should better firearms do more damage".
The rules are just going to assume firearms take off a chunk of HP like everything else does.
This post of yours is what I was referring specifically to. If regular access to magical healing is assumed an equalizer with regards to infection, then swords and guns should be relatively equivalent in lethal potentiality. If you have a world without access to magical healing, then you may consider guns a lot more damaging than swords, at least when they do hit (they may be less accurate than a bow or crossbow due to loading and targetting issues, esp. with earlier types of firearms).For surgeons getting shot was worse than being cut with a sword. If a limb was hit by a bullet, surgeons probably had to amputate because of infection. Though this was a time before antibiotics and the like were used.
But in a fantasy worlds, it probably wouldn't be as bad with the existence of healing magic and people who use herbal or alchemical remedies to disinfect wounds to the point it's almost reliable as modern medicine.
I'd also consider training here. Crossbows and Guns in real life need a lot less training than bows. You can ready an army much more swiftly if you have this type of tech, since any rabble commoner can be given a gun and be able to be relatively as effective as an trained expert, when comparing like gun models. Does this mean they really should be Simple Weapons, rather than Martial/Military Weapons, let alone Exotic Weapons? I could definitely see a laser beam gun found in the Barrier Peaks be considered an exotic weapon that takes a feat to utilise effectively without substantial mishap to the otherwise 1200s-inspired Fighter. But I'd seriously consider rebalancing dmg-dice and accuracy and additional effects/drawbacks for firearms if I was going to let them be considered simple weapons!
It's a challenging topic!