According to a knowledgeable source, multiple firearms used to protect the Media Matters founder were purchased with Brock’s blessing — and apparently with the group’s money. TheDC has previously reported that Brock’s one-time aide, Haydn Price-Morris, carried a concealed Glock handgun as he traveled with the liberal leader to public events in Washington, D.C.

http://dailycaller.com/2013/01/27/illegal-guns-david-brock-media-matters/

More on the issue:

'David Brock's Bodyguard The back story here is that Brock's former bodyguard, a man named Haydn Price-Morris, was paranoid about his boss being harmed and decided to arm himself with a concealed glock, which is illegal in Washington, D.C., where Media Matters is located. What today's Daily Caller report tries to establish is the provenance of the gun that Price-Morris obtained: According to an internal email exchange obtained by TheDC, the gun was purchased with cash in Maryland, likely to diminish the chances such a purchase would appear on the tax-exempt group’s books. Brock wound up firing Price-Morris for breaking the law by carrying a concealed gun sometimes in 2010, according to The Daily Caller: "Despite Brock’s knowledge of the guns, his organization successfully laid the blame for the lapse in judgment at Price-Morris’ feet, and he was quickly let go." The hypocrisy here is difficult to point out. Yes, it's true that Media Matters aggressively polices media outlets for perceived conservative bias on the issue of gun violence and gun control. And yes, Brock's bodyguard violated DC gun laws. But he was fired for doing so, and later a Media Matters staff member tried to make sure that Price-Morris gave the guns to the police, to be disposed of. Verdict: An embarrassing episode, but not quite hypocritical."

https://www.theatlantic.com/politics/archive/2013/01/david-brock-david-gregory-guns-dc/318978/ The gun was not registered.
https://www.scribd.com/doc/122510164/Media-Matters-firearm-emails-from-The-Daily-Caller