Lemon waded out of his comfort zone during a segment featuring conservative radio host Ben Ferguson and liberal commentator Van Jones.
Ferguson explained that "gun law says that you and I can't just go out and buy an automatic weapon," while noting that the rules covering semi-automatic weapons vary from place to place.
"But to imply that anyone can walk out and buy an automatic weapon is just not true, Don," Ferguson said.
That didn't sound right to Lemon, who was reporting on the ground in Ferguson, Mo. He claimed that after the 2012 movie theater massacre in Aurora, Colo., he was "able to go and buy an automatic weapon."
"And I've maybe shot a gun three, four times in my life — I don’t even live in Colorado. I think most people can go out and buy an automatic weapon,” Lemon added. “I don’t understand your argument there. It took me 20 minutes.”
Ferguson sensed the host's ignorance, and the ensuing exchange has had all of conservative media howling.
FERGUSON: Don, what is your definition of an automatic weapon? What is your definition of an automatic weapon?
LEMON: Well, for me, an automatic weapon is something that you can shoot off a number of rounds, a number of rounds very quickly. I was able to buy an AR-15 within 20 minutes.
FERGUSON: Don, in all due respect, you don't know what you're talking about. An automatic weapon is when you pull the trigger one time and it continually shoots off, one after another after another after another.
After Lemon accused his guest of bringing up "semantics," Ferguson told him the distinction is crucial.
“It’s a big deal,” Ferguson said. “It’s the difference between breaking the law and not breaking the law.”
The reaction on the right has not been kind to Lemon.
"CNN'S DON LEMON HAS NO IDEA WHAT AN AUTOMATIC FIREARM IS," read the headline over at Breitbart.
"This is what we're up against: People who know nothing about the subject and become indignant when you point it out," tweeted National Review's Charles C.W. Cooke.
But Lemon didn't exactly seem humbled by his pratfall. In an email to TPM on Thursday, he chalked up his error to a simple verbal slip and insisted that he does indeed know the difference between automatic and semi-automatic guns. His central point about access to weapons, he argued, still stands.
I mis-spoke in our discussion of guns with Ben Ferguson ... I do know the difference between a semi and automatic weapon. And it doesn't change the point I was making - that sales are through the roof and we as a nation should decide if that's a good thing or not.