i put my gun at coords 0,0,0 in max,
and in fpsc, it seems that 0,0 in the horizontle and verticle positions represent the very center of the screen, and work like a normal x and y graph, but forward at zero seems to be substantually behind the player.
and in fpsc, for my gun to appear, it took these coordinates in the gunspec:
horiz = 15
vert = -24
forward = 47
its really weird system.
another thing you might want to be aware of, that most people dont figure out because theire guns are symetrical, is that it flips the gun along the x axis.
i didnt even notice it until i put hands on my guns.
hope that helped