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If an individual finds an 'undeclared' family weapon in France it must report it to the authorities; depending on which type/category of weapon it is, the person may be able to keep it (e.g. a hunting rifle or derelict general issue rifle), or file for permission to detain it personally (e.g. a US Carbine) or turn it over (e.g. a Panzer Faust). Permissions are never to be taken for granted and are delivered on a case-by-case basis.
Failing to declare a 'family' weapon is considered a crime and subject to prosecution; as in the US, the French Govt. offers regular amnesties, probably because people are still slow to declare their undeclared weapons.

Louis