Over at Legal Insurrection you can read the email that the DC police sent to NBC telling them that they could not legally bring a high capacity magazine into DC for their news segment. This means that David Gregory and NBC did not unintentionally break the law: they knew that they were doing something unlawful, and went ahead anyway. A picture of a high capacity magazine wasn't enough.
If there is no criminal prosecution, it raises an interesting argument if someone else is prosecuted for violating D.C.'s high capacity magazine ban. What makes David Gregory special?