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.
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.
There is a man under the second bridge with a FG42 scoped, when you get that gun you will get ammunition every time you kill someone. The gun starts with 96 Ammo and not including melee you can kill 150 people with it. When the 2nd mission is completed you will retain the gun for the third mission but from the 3rd mission on you must save the gun to keep it.