Confusing
This game could have done with a brief tutorial. Starting with a powerful, wide-blasting gun is not a good idea, especially when you don't know how to switch when it dies.
It's a nice game, but you should explain that it is full mouse control before throwing the player into the fray, where they are least likely to think.
Another couple of options I'd recommend:
A pause button would be wonderful.
Quality control might be good too.
If any of these are in the game, but I missed them, then disregard this comment. ^_^