GnomeWorks
Adventurer
Despite the 21 foot rule, infantry units throughout the world use firearms.
It's almost like there's a difference between military contexts -- where you generally have a lot of combatants on either side -- and an adventuring context, in which the 21 foot rule would matter.
unless I'm caught flat footed, if I've got a flintlock in my hands I'm confident I can shoot that goliath before he gets close enough to hit me with his sword.
Your explicit example was blowing someone's head off with a gun. That's considerably different from "I can shoot [the target]." Hitting someone with a gun is significantly easier than completely neutralizing them as a threat in a single shot.
It's easier to shoot someone than it is to stab them.
The same can be said for crossbows.
Odds are the guy with the gun is going to win though.
The point stands that those odds are influenced by a variety of factors. A complete novice with a gun against a trained MMA fighter might not even figure out how to flick the safety off before getting his teeth kicked out.
And that's not even taking into account stuff like levels and whatnot.