Actual damage dealt is not the value in the gun description. The gun description shows only the base damage value of the gun + modifiers. This is done to give the player something to compare guns.

In current 1.13 this base value is calculated like:
- get base value from weapon
- multiply by type multiplier from Item_Settings.ini
- add bonuses from attachments
- multiply by GUN_DAMAGE_MODIFIER percent from Ja2_Options.ini

To get the damage dealt there are multiple calculations done that depend on many things, for example gun range to shot distance ratio, armor piercing modifiers, HP ammo, bonus for high hit chance and if it's a critical hit or not. If you want to know precise formulas you will have to dig through the code.