In this video I show you a road to farm tin cans, which you can then recycle into Steel and Lead, who are the most common and essential components for most ammo in the game.

This video is captured on a Custom World with invincibility and infinite ammo settings to both ensure that all the spawns are available, and also to cut short the length of the video.

In a regular server and depending of your build, you should be able to do this in 15 to 20 minutes.

Note that since the tins cans are public items, they may be not spawned because another player as already taken them, it's not a problem, just hop servers.

Do this once or twice per day and you should pretty quickly accumulate enough material to craft yourself a comfortable reserve of ammo for a while.