During the 1960's it wasn't illegal to metal detect on Shiloh or Civil War Battlefields. Only within the past 25 years has Corinth become under the protection of the NPS. Relics found on private land belong to the owner of the land. Unfortunately NPS employees at Shiloh in the past have pilfered many of the relics turned into the NPS. With the PC attitude Civil War relics, except for a few exceptions, have decreased in value. If many of the NPS liberal employees had their way, they'd prevent visitors from coming to the parks for any reason as they believe they have the God given right to exclusively use the NP system as they see fit instead of the park for the use and study by the taxpayers.
As for NPS or the government trying to confiscate personal relic collections, they have tried to in the past unsuccessfully but I feel in the future they will try again.