Yep, the main concept and low poly shape seems a good start.
It's the one I started and lost with my hardrive ;_; (but it wasn't that good, so nvm)

https://lh4.googleusercontent.com/-hQfZ ... -copie.jpg
Things that won't appear on screen (like handle or stock) don't necessary need to be overly detailed, as they will be hardly noticed. Better keep poly for visible parts. Also working with the hand model and 1rst person view help to keep things proportionate and close to how it will looks in game.