Firearms is a unique Van Buren skill, meant to replace the classic Small Guns and Big Guns skills. It was also called Marksmanship early in the game's development.

J.E. Sawyer quote:

I'm now more in favour of marksmanship than having two firearm skills.
If Small Guns, Big Guns and Energy Weapons are combined into one, wont it make being a dedicated combat specialist too easy?
A dedicated combat specialist in Fallout 1 or 2 could put all his or her skill points into one or two combat skills, too (Small Guns/Energy Weapons, for instance). Of course, since (with the exception of the beginning of Fallout 2) ammo grew on trees and fell out of the sky, Unarmed and Melee were only useful as amusing alternatives. That's why I keep talking about having lower amounts of ammo and fewer firearm-using opponents.