I have seen 20" but no shorter. Not sure what you are trying to accomplish but maybe a double or triple header baby arm would work for you or a side arm?
In situations where I need to mount a camera at the end of a superboom, it works pretty well as long as it's only a single camera and not much else. Sometimes I need to rig two cameras together though on the same pin, and it is not at all up for the task, because if the weight isn't evenly balanced the rig can spin a bit.
There are better ways to rig an overhead camera if the goal is stability and safety especially if you only need 6-12 inches. C-stand grip arms induce vibrations every time to touch the camera, these can take 30-45 seconds to calm down.
