For Metal, Nuts and Bolts go in suburbs, it is easy to collect by breaking the cars and other metal items like a washing machine or a grill, etc. While in woods you will have to look for a cave to collect metal or tracks, machines, etc. And if you are in no mood to explore then try doing the expedition mission. You can get large amount of supply in it. So in this way you can gather items first and then craft ammos from it. There is also some second delay while crafting so keep a watch, doing this in a crowded enemy area can be lethal.