From Merriam Webster Online:
Definition of WEAPON
1
: something (as a club, knife, or gun) used to injure, defeat, or destroy
Definition of FIREARM
: a weapon from which a shot is discharged by gunpowder usually used of small arms
You can be as PC about it as you want but a gun IS a weapon.
Note that your definition proves my point. "something (as a club, knife, or gun) used to injure, defeat, or destroy"
Note the verb "used." A gun sitting on a table is not being "used to injure, defeat, or destroy" therefore it's not a weapon until it is used as such.
Please understand that this is not about political correctness. It's more about lawmakers and laws. Some people get squeamish when they hear that people are walking around with weapons in their pockets. Maybe less so when using the term firearm.
George Zimmerman just happened to be carrying a gun when he was forced to deploy it as a weapon to prevent his skull from being shattered on a concrete sidewalk.