There are a variety of factors that can prohibit you from owning firearms. If you’re convicted of a violent crime, like assault, domestic abuse/violence, a dishonorable discharge from the military, or being in this country illegally. That’s the most obvious one, and it also tops the list of the reasons why someone was rejected for a firearms purchase through the National Instant Background Check System. The FBI is reporting that a record number of illegals tired to purchase firearms this year. Over seven million illegals tried to purchase guns. Those who were adjudicated for mental health were the runners up in the prohibited category, with over 5.6 million rejected NICS checks (via Washington Examiner):

The National Instant Criminal Background Check System said that it rejected 7,836,600 planned purchases from “illegal/unlawful alien” as of 2018. And that was at the end of November. Christmas purchases of guns are typically high and were not included.

That number has been increasing in recent years.

[…]

The full list of prohibited categories and NICS catches:

Illegal/unauthorized alien: 7,836,600.

Adjudicated mental health: 5,637,317.

Convicted of a crime punishable by more than one year or a misdemeanor punishable by more than two years: 3,833,213.

State prohibitor: 1,267,285.

Misdemeanor crime of domestic violence conviction: 174,388.

Under indictment: 128,214.

Protection/restraining order for domestic violence: 74,014.

Renounced U.S. citizenship: 46,346.

Unlawful user/addicted to controlled substance: 46,320.

Dishonorable discharge: 16,485.

Fugitive from justice: 1,919.