According to the FBI, over 1.5 million background checks on customers were requested by gun dealers to the National Instant Criminal Background Check System in December. Nearly 500,000 of those were in the six days before Christmas.
It was the highest number ever in a single month, surpassing the previous record set in November.
On Dec 23 alone there were 102,222 background checks, making it the second busiest single day for buying guns in history.
The actual number of guns bought may have been even higher if individual customers took home more than one each.
Explanations for America's surge in gun buying include that it is a response to the stalled economy with people fearing crime waves. Another theory is that buyers are rushing to gun shops because they believe tighter firearms laws will be introduced in the future.
The National Rifle Association said people were concerned about self defence because police officer numbers were declining.
A spokesman said: "I think there's an increased realisation that when something bad occurs it's going to be between them and the criminal."
But anti-gun campaigners said those who already owned weapons were simply hoarding more of them due to "fear-mongering" by the NRA.
A spokeswoman for the Brady Campaign to Prevent Gun Violence said: "The research we've seen indicates fewer and fewer people are owning more and more guns."
Dave LaRue, of Legendary Guns in Phoenix, Arizona, said Christmas sales were up 25 per cent on the previous year and ammunition sales were also "brisk".
He said: "There are a lot of people concerned about pending gun legislation and the sense about the current administration. People think future availability will be limited and there's a feeling of get it while you can."
The record for gun sales in a single day was set in November, on the day after Thanksgiving, when 129,166 background searches were carried out on customers buying weapons.
Since the near-fatal shooting of congresswoman Gabrielle Giffords by a deranged gunman in Tucson, Arizona last January there have been increasing calls for tighter gun control. Miss Giffords survived being shot in the head with a semi-automatic handgun, and six other people were killed.
- Americans buy record numbers of guns for Christmas