DayZ stable branch version 1.16.154567
Rubber slugs have a base damage of ~14. I believe that this itself may be unintentional but even if it isn't, their damage is not consistent.
I tested rubber slug damage by repeatedly shooting an unarmoured dummy in the chest on a test server and observed that occasionally rubber slugs did no damage and other occasions they did ~40 damage and this variance occurred while all other variables were kept the same. I was surprised when I first observed an instance of the rubber slug doing ~40 damage so I tried to replicate it and after about 3 minutes of shooting dummies with rubber slugs I replicated the ~40 damage.
In 'Additional Information' I have linked screenshots of the 3 different damage outcomes I observed (0, ~14, ~40) and a server log for an instance where it did ~40 damage