FBI gun background checks are on track for a record year.
The bureau processed 19,827,376 requests sent to the National Instant Criminal Background Check Systems, or NICS, through Monday.
That's a 6% increase from the same period last year. It's also 4% more than the same period in 2013, which is the record year for background checks.
Mass shootings, like the Thursday attack in San Bernardino, often cause a rush in sales.
Background checks are an approximation, but not a direct indicator, of gun sales. Anyone buying a gun from a federally licensed dealer must submit personal information to the FBI. The buyer must state whether he or she has been convicted of a felony, uses illegal drugs, or has faced prosecution for domestic violence.
Most purchases are conducted by federally licensed dealers and most buyers pass background checks. This is why background checks are considered to be the closest nationwide measure of gun sales.
Past mass shootings have tended to drive gun sales, partly because buyers want to protect themselves, and partly because they're concerned that the shooting may prompt more restrictive gun control measures.
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Gun sales were already on the rise before Wednesday's shooting.
Last month, Black Friday was the biggest day ever for background checks, with a total of 185,345. That beat the prior record for a single day, which was on December 21, 2012, one week after the mass shooting at Sandy Hook Elementary School in Newtown, Connecticut.
Currently, 2013 stands out as the year in which the FBI conducted the most background checks -- 21,093,273.
The Newtown shooting prompted a months-long rush of background checks that stretched into 2013. That was also the year that President Obama tried, and failed, to get a gun control bill through Congress.
Mass shootings have become a regular occurrence in the U.S.
In San Bernardino, 14 people were killed and 17 wounded. The shooters, who were killed by police, had two AR-15 style rifles and two semiautomatic handguns that were legally purchased.
This followed another recent shooting at a Planned Parenthood clinic in Colorado Springs, Colorado, where a man faces charges for killing three people and wounding nine on November 27.
