Ok, I can reproduce if I don't fullscreen the game. We'll add this to our backlog.
If you'd like to try again, the blue arrows in the bottom right to fullscreen the game should get it to run correctly!
Sorry to hear you were unable to do that, were you unable to place any guns at all, or did this effect you in the middle of the game?
There are definitely some bugs with that part of the UI we're still working out, hopefully you were still able to experience some of what our demo has to offer so far!