The 2^nd Amendment to the United States Constitution states:

A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed.

However courts and legislation have restricted some individuals “right” to own firearms. 

Restrictions on Some Gun Owners

Federal law outlaws the possession of firearms or ammunition by several categories of people, including:

• convicted felons
• anyone who’s been convicted of a misdemeanor for domestic violence or is under a domestic violence restraining order (but only if the order protects an “intimate partner” or child from physical force or threats)
• people who’ve been committed to a psychiatric institution or labeled mentally ill under a court ruling
• undocumented immigrants and those in the country under nonimmigrant visas
• illegal drug users, and
• former military members who had a dishonorable discharge.

(18 U.S.C. § 922(g).)

Many states prohibit or restrict gun possession by other groups of people, such as stalkers and people subject to other kinds of restraining orders, minors, juvenile offenders, and those convicted of alcohol- and/or drug-related crimes.

Several states also allow courts to order some people to give up their guns temporarily if they pose an immediate risk to themselves or others (under so-called "red flag laws").