I knew that NICS background checks are done on people who want to own a firearm or explosive. (see https://www.fbi.gov/how-we-can-help-you/more-fbi-services-and-information/nics)
You mean that more than half are for explosives, or are there other categories?
For a while the number was about 60%, with Illinois and Kentucky responsible for about 75% of those.
Those two state would run their entire permit list every month, rain or shine.
Over half a million permit checks and rechecks, just from those two states, every month.