What can I use as a thread "tightener" compound for a free-running bolt?
Based on the clarifications, I would recommend using a reusable thread-locking liquid.
Here is a link to one I found that may work. I have not used it.
Vibra-Tite VC3 Threadmate  http://www.vibra-tite.com
(I am in no way affiliated with the Vibra-Tite Company)