Data: FBI; Chart: Andrew Witherspoon/Axios Firearms background checks in the U.S. hit a record high in 2020.The big picture: This past year took our collective arsenal to new heights, with millions of Americans buying guns for the first time. That trend coincides with a moment of peak political and social tension.Be smart: sign up FREE for the most influential newsletter in America.By the numbers: According to FBI statistics, more than 39.5 million firearms background checks were processed in 2020, by far the most since the agency began keeping records in 1998. * Nearly 4 million checks were processed in December alone — the single busiest month ever — and the total for 2020 was almost 40% higher than in 2019, which had been the previous record-holder. * State after state after state — both those with lax firearms laws and those with tighter restrictions — reported record gun sales.The catch: FBI firearms background checks are an imperfect metric for gun sales — they're also carried out during applications that don't involve firearms purchases, like for a concealed carry permit — but with no other federal data for tracking sales, they're considered the best existing indicator.Be smart: While background checks have increased nearly every year over the past decade, what happened in 2020 goes beyond the sheer scale of the surge. * In the past, sudden spikes in firearms sales were generally prompted by fears of tightening gun control laws — usually in the aftermath of a mass shooting event or the election of liberal Democrats — but 2020's surges seem to be connected to more general fears. * March — the month when pandemic restrictions snapped into place — saw what was then the biggest month on record, only to be eclipsed in June during the peak of the protests around George Floyd's murder. * Nor was 2020's increase simply a matter of existing gun owners expanding their arsenals. The National Shooting Sports Foundation (NSSF) estimates nearly 5 million Americans bought a gun for the first time last year. * And it's across the political spectrum. As Politico reported in October, a number of those new gun buyers include Democrats and voters who said they were opposed to President Trump but still feared for their own safety.Of note: A fascinating study published in JAMA last week surveyed thousands of Californians and found a notable rise in fears about violence because of the pandemic — especially worries about lawlessness — with a resulting increase in firearms acquisition and less secure storage of guns. * According to statistics compiled by the reporter Jeff Asher, the murder rate was up 36.7% in 57 reporting areas through September, with big cities like Los Angeles, Chicago and New York all seeing increases higher than 30%.Context: These numbers stand against the backdrop of the insurrection at the Capitol on Jan. 6 and more general fears about growing political violence. * As more details are reported about the insurrection, it's becoming clear that police officers' initial reluctance to engage the rioters at the Capitol was likely due at least in part to the fact that many of them were clearly well-armed. * While data has yet to be compiled for the first two weeks of 2021, gun shops are reporting further spikes in sales following the riots.What they're saying: "The potential for chronic political violence in the United States is rising," says DJ Peterson, president of Longview Global Advisors. "You have a large number of guns and ammunition in public hands, and a large share of the population has military or paramilitary training."Yes, but: Whatever political violence is in store, the U.S. is unlikely to end up like other countries that have been torn apart by civil war, in part because the American military — even though it has real problems with right-wing extremism within its ranks — remains nonpolitical and capable of countering any insurrection. * Still, the combination of many guns — an estimated 434 million firearms are in civilian possession, more than one for every American — and collapsing public trust will be a combustible one for the foreseeable future.Support safe, smart, sane journalism. Sign up for Axios Newsletters here.