The visual indicator for traverse angle, as well as the actual model of the weapon, suggests that rivet rifles should be able to point quite far downward, about 15ish degrees? In practice however, they are unable to aim anywhere near as far down as they should be, more like 5 degrees or so.
This is pretty debilitating for rivet bots with guns mounted on both the top and bottom (or side mounts), as a target even a little under the horizontal plane of the weapon can't be fired at.
Hard to get good pics of but I tried.