When I saw that the fire rate upgrade was a flat reduction in seconds, I knew immediately what I wanted to do. I stuck around long enough to get the fire rate all the way down to 0.1 second but it looks like that is the floor. I was hoping to get a bullet laser out of the deal, but its probably for the best that this safeguard is in place. I have to imagine the code wouldn't really be happy finding a negative number there. In any case, it would probably be fine to bump up the strength of the upgrade to give a bit more benefit up-front. The ships that shoot bullets that collide and eat your own bullets can be pretty taxing early on when bullets per second is scarce.