Sorry to break this to you, but a while back a mod explained the reason why they can't add new guns to the main game is because of how they handled memory management for weapon models. Long story short, they made each weapon take up too much space and didn't leave enough to add new weapon models. Only way DLCs like Gangstas in Space could add new weapons was a workaround they figured out (you'll notice practically none of the regular guns were available in GiS), but that same workaround makes it impractical to use in the main game.
Let's hope they learned their lesson for SR4.
