Spend at least 25,000 Overcharge at the stores. Go to Two Hat Jack and purchase Guns and Ammo. You will eventually spend up to 25,000 Overcharge. Overcharge can be found by killing OD and completing missions. Also play online to earn extra Overcharge. I have been playing for a couple hours (campaign and online) and have already earned 13,000 Overcharge. Bought a gun and some ammo and the Xbox Achievement tracking bar is over half way full. So just keep playing and collecting Overcharge and spend it. It's as simple as that.