I don't need guns to one-shot people. But I do want them to feel like one of the most threatening weapons out there. I think PF does a good job with that, letting guns bypass armor at close range, but not dealing substantially more damage than a sword or crossbow.
The 4x crit multiple also makes the firearm feel more dangerous, even though a threat is only a 5% chance. I think Pathfinder balances things pretty well.
The three levels of Firearm frequency are also an interesting approach. Guns just coming in are an exotic weapon, so not a lot of characters are good with them. At "commonplace", they are Martial weapons, so warriors may consider these instead of their Longbows. Once they're "everywhere", they're simple weapons, so they can be "the great equalizer".