I could see an error rate of less than 1% but not 27%, that is unacceptable.
I wonder what types of mistakes they are though. Is it the fault of the FBI in their record keeping? Is it the fault of other jurisdictions that feed data into the national database? For all the ones that get overturned on appeal, why do they get overturned?
I know a guy who was applying for a job but an open burglary case from 20 years prior flagged him. The flagging was done accurately but the detective handling the case had never closed it out properly after the owner didn't want to file charges and he was released. It wasn't for a gun but I think a gun purchase would have brought up the same flag. In this case it wasn't the fault of the national database but a detective not doing his job properly.
Based on previous research, I read the majority of denials was based on incorrect cross-referencing of applicant's names with other ineligible people's names.
If John Smith is an upstanding, law abiding citizen, but the FBI has 1500 John Smiths in their databases, they may accidentally pull the wrong John Smith's information -- someone with felony convictions or domestic abuse charges.
The FBI would rather deny the purchase in those cases and spend additional time unraveling the truth after the applicant complains rather than work quickly in the 3 days the law provides to investigate. While the 3 day maximum limit the FBI has to issue a rejection or approval of applications helps buyers in most cases, it can also cause "false positives" if there is a problem.
The SC Church shooting happened in part because the FBI approved a purchase by crossing the 3-day window even though they saw discrepancies in their data. One report was clean, while another showed treatment for mental illness. Rather than "deny and figure it out afterward," the FBI allowed the application to be approved automatically at the 3 day point.
The NICS system is far from perfect, and asking what the specific cause of the denials were really doesn't help anything. All the problems have been identified already in the FIX NICS project. We need the actual solutions to be implemented.
http://fixnics.org/#&panel1-1