The best way to get the sniper badge is to aim for enemies heads @@. you should get 550+ hs. If you're having trouble then play the game with only the magnum. That pretty much enforces 500+ headshots or you won't survive.
4 stars overall, but honestly it's amazing for a first game. You have a very developed understanding of what makes a game like this interesting/fun/balanced. I look forward with great interest to your next project.
Awesome game. Excellent quality, gameplay, and even some replay value I think. Try beating the game without using either the hunting rifle or the Barrett for a challenge.