You need a registration certificate to be in legal possession of a spent casting?
Again, there is NO evidence you need to be. It is bad reporting. It is not in the DC code or DC case law. There is zero evidence of a single arrest.
A hypotheticals question, which may or may not have been accurately reported, is not a law.
In DC you are required to have firearms registration (it is a plastic card like a CCW permit but not for CCW) in possession or be in the process of acquiring a foid, while in possession of ammunition. The question likely came up because some other jurisdictions and courts apparently ALREADY define spent casings as ammunition.
As I noted there are much more salient problems with DC ammo provisions and gun laws than this made up problem