In "the forest", level 7, when you get to the point that huxley yells, "Dix we've got a live one here" restart the checkpoint. If you kept the springfield, you will now have the maximum ammo for it.
Note that the ammo is not unlimited.

Choose chapter select and hold down the arrow button and press right, right, left, left, 2, 2.

Go to the Scene/Level Selection, hold + and then press: left, left, right, right, 2, 2

Successfully complete the game to unlock interviews with veterans: Gordan Boulton, sgt. John D, "Bud" Hawk, and sgt. Stan Markut. Enjoy :)

When you are reloading your weapon you can cut part of it out, look at the amount of ammo in the clip, when it's at the maximum number hit UP on the D-Pad to switch to your other weapon then hit UP again(quickly) to get back to your first weapon it's clip should be full.

Unlock all levels and bonus content- At the Chapter Select menu, press and hold the PLUS button, then using the control pad hit: RIGHT, RIGHT, LEFT, LEFT, 2 BUTTON, 2 BUTTON quickly.

Happy gaming =p