Instead of using GameObject.SetActive to play the guns, use ParticleSystem.Play(). Why? It is because when you let go of the space key, the particles that were already emitted before letting go of the space key disappear too. Didn't Ben and Rick cover how to solve this problem in the Argon Assault section of their Complete Unity 3d Development?
Nice game. I like the simplicity, movement, and the polished graphics of your game. I think one thing to work on is adding breaks. Maybe to add some more fun, you can add turbo boosts.
