The FBI's National Instant Background Check System processed 1,795,102 firearms related applications in September.
That represents a new record: 335,739 more checks than the previous September high set in 2012, or a 23 percent increase.
The number of checks done in a particular month is considered a reliable gauge of how many gun sales have occurred since background checks are required on all sales made through licensed firearms dealers.
The actual number of sales is likely higher since multiple firearms can be sold to the same person by a dealer under a single background check. The numbers also do not account for sales between private parties that do not require a background check.
The Second Amendment Foundation said Democratic efforts to impose new gun control measures were fueling the sales increases.
Hillary Clinton, democratic presidential frontrunner, is promising to impose stricter gun laws if elected.
"We are seeing new record highs in gun sales due to the increased anti-gun rhetoric from Democratic candidates like Hillary Clinton," said Alan Gottlieb, the group's founder. "Their push for new restrictions on gun ownership is fueling gun sales."
"If they really want less guns in private hands they should consider what happens every time they open their mouths."