There's definitely a lot here to digest from both sides of the argument. I know from my studies of history that
@QuentinGeorge is -absolutely- correct that armor was effective against firearms to varying degrees, but not economically viable. Particularly after Napoleon began the era of insanely huge armies on the field of battle as opposed to the much smaller armies of earlier eras.
Agincourt, for example, saw an incredibly massive 25,000+ Frenchmen lose to 10,000+ Englishmen... But Napoleon marched around Europe with almost 700,000 Frenchmen just 350 years later.
On the other hand, I had not considered the hydrostatic lateral pressure from a bullet. And while I still do not think it comparable, it is something to consider. The reason being that the example clay shot used 78 grains of modern 300 Blackout powder which is very different from the 50-60 grains of much lower quality gunpowder that would've been available in the early renaissance. Primarily due to relative chemical purities and varying levels of mixture without laboratory equipment providing precise measures.
For now, what I think I'll do is:
1) Make Pistols and Rifles available for players to purchase but otherwise largely have them in the hands of the well off.
2) Treat all these Pistols and Rifles as Breach-Loading weapons rather than ramrod.
3) Define all ammo for such as paper-cartridge shots that are very susceptible to water damage.
4) Make that ammo -dirt- cheap.
5) Get rid of any Misfire chance. Seriously, that's not remotely fun for anyone. If a gun blows up it should be for narrative effect, exclusively.
6) Assign them damage equivalent to Hand and Heavy crossbows for Pistol and Rifle, respectively, along with proficiency and feat-interactions.
7) Give them the same load/ammunition qualities as the respective crossbows.
8) Make their fire audible from 100ft away for pistols, 300ft for rifles (mirroring
Thunderclap and
Thunderwave)
9) Make them simple weapons that someone with the "Noble" background can start with in place of a ranged weapon from their class)
10) Change their Damage Type to Bludgeoning.
11) Have a great time.
All weapon damage in the game is, after all, an abstraction placed behind a further abstraction. A combination of luck, mettle, and morale rather than a measure meat being destroyed
Probably won't use Mercer's Gunslinger... Though I might make a Rogue or Ranger subclass that does gun-tricks as a fun option.