Congress can pass whatever laws it construes as consistent with its powers, but the U.S. Supreme Court has the final say in interpreting constitutionality.
In 2008, the court's decision in District of Columbia v. Heller upheld a citizen's right to keep a handgun in his or her home for self-defense. However, as Justice Scalia noted in his majority opinion, the right to arms is not unlimited, but leaves room for "longstanding prohibitions" on possession by felons or the mentally ill, or on carrying firearms in schools or government buildings, or even on "dangerous and unusual weapons" [source: 554 U.S. 570].
The scope of Heller was limited to federal jurisdictions, but the following year, in McDonald v. Chicago, the Supreme Court extended its interpretation to encompass state and local laws [sources: 554 U.S. 570; 561 US 3025; Liptak; The New York Times].
The highest court in the land left it to the lower courts to reconcile these broad rulings with existing laws, and in 2012 the 7th U.S. Circuit Court of Appeals -- which covers Illinois, Indiana and Wisconsin -- rose to the occasion with Moore v. Madigan. In interpreting Illinois' law banning gun possession in public, the court ruled that the right to self-defense upheld in Heller and McDonald extended beyond the home to public places where, according to the Circuit Judge Posner, threats were more likely to occur [source: 7th Cir. 2012].
Although it remains too early in the game to predict the ultimate impact of Heller, McDonald and Moore, the decisions have kindled hope in some offenders, particularly the nonviolent variety, of one day regaining their gun rights [source: Luo].
Before these rulings, a few key pieces of legislation established the limitations on gun ownership. The federal ban on felon gun possession was solidified by the Gun Control Act of 1968, spurred to passage by the assassinations of both Sen. Robert F. Kennedy and Martin Luther King Jr. that year [sources: Finney; ATF].
The Federal Firearms Act of 1938 (FFA) -- pushed for in response to post-Prohibition gangster violence by, among others, the National Rifle Association -- attempted an earlier felon ban but was so riddled with loopholes that it was virtually impossible to prosecute anyone under its statutes [sources: Kakutani; Zimring].
Before the FFA, the National Firearms Act of 1934 (NFA) established gun registration and dealer licensing as part of an effort to curtail weapons favored by gangsters. However, because it required registration of even illegal firearms, it violated the Fifth Amendment protection against self-incrimination, according to the Supreme Court decision Haynes v. United States. Congress overhauled the law in 1968 and, to this day, certain weapons, such as short-barreled rifles and grenades, are classified as Title II or GCA weapons [sources: 390 U.S. 85; Kakutani; Lacey-Bordeaux; NFA; Zimring].