There are also high level special weapons designed into the game. These do occur at random drops but come preassigned with set values and names from the dev team. So not all items are randomly generated.
Tye
Ok, after looking into it, the gunz work like this. The game has 3 "manufactures" that create weapons and sells them. Each manufacturer specializes in certain kinds of weapons. Lets say one company specializes in revolvers, sniper rifles, corrosive, and crossbows. Their is a "A.I." that shuffles these and sticks 1 to X on a gun and gives a weapon of that random configuration.
Example: Early on I would image this is limited to say 1. So you get stock revolver, stock sniper rifle, etc. Then you reach say level 8 and now the X is a 2, so weapons you get are sniper rifles with revolver drums (faster reload), crossbows that shoot corrosive bolts, etc. So each level you earn that unlock what and how many the A.I. can random shuffle together.
This allows the game to limit what you have access to and why they claim so many weapons are in the game. You got 3 companies, 5 specialties, 25 other gun attributes they share, each gun has 5 levels of general performance, and 3 levels of your personal alterations.
I am sure I have the system right, but my numbers are just examples. They are being kinda tight liped about exact features for now. Hell they have only confirmed like 3 vehicles.