Sure, the PHB has firearms artificially frozen there,
What makes you believe that they are "artificially" "frozen"?
It is unknown whether most D&D worlds have reached the level of metallurgy that allows casting or boring firearms to be manufactured cheaply. Why do you feel that D&D worlds being at this level of technology is "artificial" compared to a different level of advancement?
and doesn't address how they would impact armor.
Usually with a loud "Clang!" . . .
The PHB
does address how D&D firearms impact armour: They have a chance of bypassing armour completely based on the attack roll. Given that even heavy cloth was able to stop the more advanced Napoleonic muskets, and shields were still used effectively against firearms, why do you think that they would do so differently from any other attack?
I mean, if D&D firearms having to check to see if they penetrate armour hurts your verisimilitude, then surely the fact that they can be fired every 6 seconds at least also misses your V-spot?
Either you have realistic firearms which are currently inferior to the existing missile weapons in the rules and so wouldn't impact the setting much, or you accepts that the firearms rules, alongside all the other rules, are an abstraction, and so "realism" is not required.
D&D opts to ignore it. Which is fine. D&D armor is hardly realistic. The kinetic impact of 21 foot tall, 8,000 lb giant's axe would reduce a human to red mist; plate armor or no. The same armor would roast him alive when bathed like dragonfire like a baked potato in tinfoil. Bullets are child's play when you consider that.
I've watched enough action flick where the hero "takes a stray" shot to the arm and shrugs the wound off one scene later to accept D&D bullets are slightly better than bolts/arrows and not the near-instants-fatality machines they are IRL.
I mean the belief that bullets are "near instant-fatality machines" is just as hollywood-driven as people being able to shrug off wounds for a while, if not more so.