What do you mean? don't all weapons work with it already? unless you mean the aiming system the guns should make a lazer to show where the player is aiming
you mean like simple guns that don't aim and have horizontal only spread?
This could be modified using a script check to see if the person is like a Top-Down player, if so, any projectiles created by that player that have any vertical speed will be removed.
http://forum.blockland.us/index.php?topic=61505.0