"ISIL training videos are telling lone wolves that the easiest way to buy a combat assault weapon in America is at a gun show," said O'Malley, the former Maryland governor who is lagging in the polls. 
"And it's because of a flip-flopping political approach in Washington that both of my two colleagues on this stage have represented there for the last 40 years."
Sanders, the senator from Vermont, interjected: “Whoa, whoa, whoa, whoa. Let's calm down a little bit Martin. … Let's maybe have some rules here, commentators."
The former secretary of State jumped in as well, saying, "Yeah ... Let's tell the truth Martin."
Both Sanders and Clinton said they have always believed in commonsense gun control. Sanders portrayed his views as being politically heroic in a state that has many rural gun owners and virtually non-existent gun laws.
O'Malley hit Sanders for voting to give immunity to gun dealers, a subject Clinton has been attacking her main rival on.
"Secretary Clinton changes her position on this every election year, it seems," O'Malley added. "Look, what we need on this issue is not more polls. We need more principle."