Look at the manual!

Use vec_for_vertex. That should be what you're looking for.

But if you're looking to match the gun's pan, tilt, and roll with the arm's, then you'll need to do some calculations (vec_for_vertex doesn't do that automatically).

Code:
void main()
{
    cout << "I am MrCode,";
    cout << "hear me roar!";
    system("PAUSE");
}