My take on it is this:
In general, guns are not deadlier than swords. They are easier. That is the reason why people switched. It is perfectly reasonable for a sword to deal equal damage to any given gun, up until we hit explosives. I usually treat them as such in my games.
On Swords:
Even a moderately sized knife can easily inflict a lethal wound in a single stab. Anywhere in the torso is likely to be lethal or at least incapacitating, as is any sort of cut on the neck, or anything that hits an artery. Large swords can shatter bones, cut through several inches of flesh, and inflict massive internal trauma. They have the additional benefits of being quieter and more versatile in use. (you can cut ropes, food, other things, etc.) The unspoken caveat to all of these statements, however, is "... in the hands of a trained swordsman."
Swords are heavy and awkward to those who are not trained regularly to use them. You have to be fairly strong to use one properly (I know there are exceptions, I'm speaking in general here). Training becomes very important, and tactical advantages disappear quickly if the enemy knows how to handle his blade better.
On Guns
Guns are pretty much point and click. There is a limited level of technical training required to make sure one operates reliably, but once that hurdle is overcome the gun will shoot with the same force for everyone. Accuracy becomes an issue, but they have the advantage over swords in that they will "win" any exchange of blows outside of melee range. Doesn't matter how good you are with a sword, it's only a few feet long.
The damage they can inflict and its type varies wildly between models, but is more measurable and consistent since the force comes from the gun not the user.
So I treat them as equal in damage, with different narrative advantages.
