Let's look at that.
First, Article I, §8: "To exercise exclusive Legislation in all Cases whatsoever, over such District (not exceeding ten Miles square) as may, by Cession of particular States, and the Acceptance of Congress, become the Seat of the Government of the United States".
Then, Amendment II: "the right of the people to keep and bear arms shall not be infringed".
Now, this so-called "law" that David Gregory broke was not passed by Congress - you know, the Congress that has EXCLUSIVE power to legislate, IN ALL CASES WHATSOEVER?. And, this act of pretended legislation infringes on Mr. Gregory's right to bear arms, neh?
Any way you look at it, he should be in the clear.
Congress passes every gun law? News to me. Some DC goofball got this passed -
DC High Capacity Ammunition Magazines – D.C. Official Code 7-2506.01
(b) No person in the District shall possess, sell, or transfer any large capacity ammunition feeding device regardless of whether the device is attached to a firearm. For the purposes of this subsection, the term large capacity ammunition feeding device means a magazine, belt, drum, feed strip, or similar device that has a capacity of, or that can be readily restored or converted to accept, more than 10 rounds of ammunition.
Gregory lives and works there. He even called the cops beforehand and they rejected his request. He broke the law. Lock him up.