The problem, as far as realism is concerned, is not that guns are insufficiently deadly; it's that they can't kill in one shot. (The only time they can kill with one shot is when the target is "weak" enough to be guaranteed to die in two shots.)
The distribution is wrong. Death shouldn't arrive on the nth shot, with no deaths on the first shot and death guaranteed by the n+1th shot.
That's wrong for any weapon, but it's especially jarring for one-shot weapons. It breaks expectations -- both realistic expectations and action-story expectations -- when, say, a hold-up is a guaranteed non-issue, or a duel can't kill either party, or a hunter can't take down common game, or a sniper can't take out an officer.
In other areas where this problem is too jarring, we add coup-de-grace rules, or sneak attack bonus damage, or iaijutsu damage. I don't think those are a perfect fit for ordinary gun-fights, but they point to some other places where hit points and expectations don't match up well.
Regarding One-Shot Kills:
Thing about items/abilities that bypass hit points and becomes a 'one shot kill', it does come in game, but usually at quite some cost. An assassin can do it, but then you don't get to play a caster or fighter, unless you use lots more levels so you take enough rogue to qualify for the prestige class. It comes with a class/level/feat cost - actually quite expensive.
Pathfinder has the Ninja, but it's Assassinate ninja trick is a 10th level ability at minimum. Even more expensive.
20th level rogues have an assassinate like ability at 20th too - but then that's 20 levels of cost.
Vorpal weapons are +5 magical weapons, not including it's actual enhancement bonus - a very, very expensive sword. And you still have to confirm a critical hit, on a beings without immunity to crits.
So if you introduce a single weapon, that anyone can have that might require exotic weapon proficiency that offers a one-shot-one-kill capability. It's now very cheap too achieve the assassinate ability. You don't have to leave your class, your concept, or anything except possible one required feet that a 1st level character can take -
that's way too powerful.
Technically speaking a lucky shot by an untrained person (0 level commoner) wielding a knitting needle could bypass your armor, even your notice and pierce your heart, kidney or brain. If you allow firearms to have one-shot kill capability, then you have to allow all weapons that capability, because no matter unlikely it would happen, by the effects of 'reality' it could happen.
This would mean that vorpal weapons, expensive feat/class choices to achieve a one-shot kill capability is a waste of time, because anybody can win an encounter with a one-shot kill, and I think this would largely diminish my game.
One shot kill firearms should be expensive magic items like vorpal swords, otherwise it unfairly imbalances the game in a huge way.
And a sniper could by the rules easily take out that guard with a one-shot kill, as long as the guard was a 0-level warrior. If he's got less hit points that the amount of damage your weapon causes - he is dead. It's easy to one-shot killing any NPC as long as they don't have many hit points.