He shoulda declared in advance his intent to register his rifle.
Here's the procedure (from the Washington, DC website):
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Previously Owned Firearm
To register a firearm you already own, you must file an application immediately after the firearm is brought into the District. You can comply with this requirement by personally communicating your intent to register the firearm to the FRS before bringing the gun into the District. You must then file an application for a registration certificate within 48 hours after communicating your intent to do so.
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Guess the Iowa Guy didn't tell him. But then, he shoulda known.
FWIW,