I just read a online item about the event; www.Thehollywoodreporter.com
Their article states that a NBC staff member contacted a federal/ATF official & a DC Metro senior officer and got "permission" to display the 30rd alloy mag on the air. Multiple news sources have said the Washington DC police REFUSED the NBC staff request to use the real AR magazine as a prop.
David Gregory has been in hot water with Meet The Press before. He seems to have ethics & conduct issues IMO.
I miss Tim Russert. He was a stand-up guy & straight shooter(pun intended)
Fun fact: Tim Russert once made a brief cameo as himself in the NBC cop drama: Homicide, Life on the Street.
PS; A few years ago, I read a great non-fiction book about the early years of NBC's Saturday Night Live(SNL). There's a interesting bit about how a real, full auto(select fire) M-16a1 rifle that was used as a TV prop in the NBC studios became "lost" on the set. When 2 ATF agents went to Studio 8H, Dan Ackroyd ducked out & took off! True story.