State and local laws vary, of course. The federal law regulating firearm possession, tranfers, and transportation across state lines is found at 18 U.S. Code §922 (read it here).
In general, federal law prevents you from having a gun if you’re a
- felon
- fugitive
- drug addict
- undocumented immigrant
- dishonorably discharged veteran
or you’ve
- renounced U.S. citizenship
- been convicted of domestic violence
- have a domestic protection order against you
- been adjudicated by a court as mentally defective
- were involuntarily committed to a mental institution
That, of course, doesn’t stop people who aren’t supposed to have guns from having guns. It’s not like people who commit gun crimes ask anyone’s permission first. Illegal possession of a gun is an additional charge prosecutors can tack on, and some states have enhanced penalties for crimes committed with a gun, but this obviously isn’t stopping or even slowing down gun violence in the U.S.A.
Why do we have so much gun violence, when other countries don’t? This article explores some of the reasons.