At the beginning of Chapter 2-1, you will be in a garage with two Humvees, numerous breakable crates, and a machine gun. Pick up any items you wish, including the machine gun. End the game, and select "Yes" to overwrite your current equipment. Continue the game from the main menu. You will have gained your items, as well as the machine gun. From here you can sell the machine gun back for quick money, then repeat the process over again. This is an easy way to help upgrade your weapons early, as well as max your ammo out.