The number of background checks  that the FBI had to conduct hit a record-high this past month. As the Free Beacon reported:
The FBI’s National Instant Background Check System processed 1,795,102 applications  to buy a firearm 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.