It is hard to see how Gregory would not be found guilty of breaking DC's code.
The penalty is up to a $1,000.00 fine and up to 1 year in prison. He would also be required to register as a Gun Offender.
His remarks that "this is a 30 round magazine" are not hearsay and are definitely usable against him in court.
Penalties: DC ST § 7-2507.06
Magazine Ban: DC ST § 7-2506.01
Gun Offender Registry: DC ST § 7-2508.01