On Federal land in the west, what was once open to all sorts of hobbies, is now very controlled and restricted. You can hike out there, and shoot targets or hunt without a problem, but metal detecting is another story. As far as I know, there is no Federal law against metal detecting for coins or gold nuggets. Any artifact over 50 years old it is illegal to remove, so that rules out any of the WWII training areas, picking up purple bottles etc. Nevada also has laws governing artifacts and ghost towns, perhaps stricter than the Feds. In Oregon a few years ago, a couple of guys got in real trouble rooting through a WWII logging camp dump. They had some bottles and a log cabin syrup tin, over 50 years old, and ended up in court over it. And of course you are never to touch an arrowhead or other Indian relic.