Rusted gears are already hard enough to find in Arc Raiders without everybody and their nan looking for them to upgrade their Gunsmith bench. They're an oddly rare item in the post-apocalypse, even more so considering how many abandoned factories, machines, and vehicles there are topside.
Well, I've got good news and bad news. The good news is that there's a very lucrative route through Buried City that'll often give you two, three, or sometimes even more rusted gears per run. The bad news is that it's not exactly quick, what with all the breaching you'll have to do.
