Making up weapons is easy as pie. "Its a gun. It shoots people. It does 1dx damage. It has x bullets. Go."
Consulting a massive, detailed list of highly specific weapons stats and possible modifications based on carefully researched testing data and informed by the field experience of gun nuts? Kinda hard, especially if you yourself are not a gun nut.
Unless you have a player like I do, who, while not a gun-nut, is a highly technical person who is very detail oriented. I once watched him, in a GURPS game, craft the equivalent of a sniper rifle in a setting where the highest tech gunpowder would be an iron cannon, all the while using real-world knowledge to justify it. *mind boggles*
I'm not familiar with SC 2's layout or anything, but I don't know why you couldn't split the difference and have a small selection of common/iconic guns in each classification.