It works in my tests (I did run 2x clients on Linux, and I can shoot from one to another :) ).
I suspect it just looks as if it doesn't work, because there is completely no feedback when shooting (visual or audio). It is a TODO to add one :)
Ask a friend to join (or join yourself, just run the game 2nd time; you can use Escape key to get out/in of the mouse look in game). Left click when aiming at another player, it will hurt them (and 10 shots will kill them).
If it still doesn't work, let me know the operating system and how do you test :) Thanks!