It's a hot summer for the gun industry.

Background checks for gun sales spiked 11% in June compared to last year, making it the busiest June ever, according to the FBI's background check data.

Last month the FBI conducted nearly 1.53 million background checks, which are required for all in-store purchases, but not for all sales at gun shows or any transactions between individuals. (Federally registered dealers who sell at shows must conduct background checks, but not all sellers are federally registered.)

That's the highest volume of checks in June since 1999, when the FBI started keeping track.

Gun background checks have climbed annually since 2003 with the exception of just three years, one of which was 2014, indicating a clear jump in gun sales.

Michael Bazinet, spokesman for the National Shooting Sports Foundation, a gun industry group, said the increase is driven in part by the influx of more women and first-time gun owners into the market.

Mass shootings can also trigger sales spikes, because they tend to spark more talk about and fear of tighter gun controls. Background checks for gun sales jumped 40% in December 2012, the month of the Sandy Hook massacre.

The June 17 murder of nine black parishioners in a Charleston, S.C. church prompted President Obama to again address the need for more gun control, but experts doubt there's any connection between that tragedy and sales.

In fact, May 2015 was also a record month for background checks year-over-year.

"There was a real public dialogue about gun control" after some shootings, says Urban Institute criminologist Sam Bieler. But now, he says "the focus has been on the Confederate flag rather than on new firearm regulation."

Correction: An earlier version of this article incorrectly reported that no background checks are conducted for sales at gun shows.