Is it possible to round up 16 million AR-15s and AK-47s?

Democratic presidential candidate Beto O'Rourke's recent vow to take away people's AR-15 and AK-47 rifles raised one big question: How is it possible to round up the millions of such guns that exist in the United States?   

Mass shooters exploited gaps, errors in US background checks

Most mass shooters in the U.S. acquired the weapons they used legally because there was nothing in their background to disqualify them, according to James Alan Fox, a criminologist with Northeastern University who has studied mass shootings for decades.

O'Rourke calls for mandatory buyback of AK-47, AR-15 rifles

Democratic presidential hopeful Beto O'Rourke says a mandatory government buyback program for AK-47 and AR-15 rifles is needed because other gun control measures don't do enough to remove weapons already on the streets.