End Purchase of Firearms by Dangerous Individuals Act of 2015

End Purchase of Firearms by Dangerous Individuals Act of 2015 This bill requires each state to establish a reporting system through which mental health professionals may report information about an individual committed to a mental institution who poses a danger to himself/herself or others or an individual who communicates a serious threat of physical violence against another individual. The legislation requires states to provide mental health records to the National Instant Criminal History Background Check System (NICS) for use in determining whether a person is disqualified from possessing or receiving a firearm. It sets forth provisions to ensure maintenance and removal of records, inclusion of an appeals process, and re-evaluation of disqualifications for individuals under age 18. The bill also requires states to establish a process related to temporary involuntary commitment of individuals at mental health facilities. The Department of Justice (DOJ) must report annually to Congress on states' progress implementing the reporting systems. The bill authorizes DOJ to reduce a state's formula grant allocation under the Edward Byrne Memorial Justice Assistance Program for failing to comply. It directs the Department of Health and Human Services (HHS) to establish guidelines for state reporting systems and state processes regarding temporary involuntary commitment. HHS must also establish a process to certify state compliance. The bill amends the federal criminal code to expand the prohibition against selling firearms or ammunition to persons committed to any mental institution to include persons who communicate to a mental health professional a serious threat to commit an act of violence. Miscellaneous provisions clarify the treatment of individuals temporarily involuntarily committed by a state process, limit civil liability for mental health professionals who fail to report, establish criteria for determining dangerousness, and require a comprehensive study of state statutes that require mental health professionals to directly report certain individuals into state firearm prohibition databases.  The bill replaces various statutory references to persons adjudicated as a "mental defective" with persons adjudicated as "ineligible due to disqualifying mental status."