The Second Amendment to the U.S. Constitution says Americans have the right to bear arms, but the sale, possession, transportation and use of weapons are covered by a host of local, state and federal regulations.
Gun laws have come under scrutiny after 26 people were killed in a shooting rampage at a Connecticut school on Friday. The suspected shooter, Adam Lanza, is believed to have stolen the guns from his mother.
Some U.S. politicians, including President Barack Obama, have called for tightening U.S. gun laws, including a ban on so-called assault weapons.
Guns bought from licensed dealers are subject to federal and state laws and frequently require background checks and waiting periods. Buyers purchasing firearms from private sellers can usually avoid such checks and delays.
Here is an overview of the patchwork of federal and state laws applying to firearms:
** U.S. federal law only permits a "qualified law enforcement officer" with proper agency-issued identification to carry a concealed firearm. But every state in the nation, except Illinois, now allows residents to carry concealed weapons. An appeals court earlier this month ruled the Illinois law banning most people from carrying handguns in public is unconstitutional.
** The 49 states that permit the concealed carry of firearms are broken up into two groups: "shall issue" states and "may issue" states.
In so-called "shall issue" states, officials are required to issue a permit to anyone who meets minimum requirements.
Once a permit is issued, the permit holder may carry a loaded, concealed firearm in a public place -- although a number of states prohibit concealed weapons in specified locations.
There are 35 "shall issue" states, including Florida, Indiana, Michigan, New Hampshire, Ohio, Pennsylvania, Texas and Washington.
In the so-called "may issue" states, including Alabama, Connecticut, Hawaii, Massachusetts and New York, officials have discretion to grant or deny the permit, based on various statutory factors. Most "may issue" states require applicants to show a bona fide need to carry a concealed firearm.
In addition to the "shall issue" and "may issue" states, there are four states -- Arizona, Alaska, Vermont and Wyoming -- where no permit is required to carry a concealed weapon.
Of the 49 states that allow concealed weapons, fewer than half require applicants to demonstrate basic knowledge of firearm use and safety. They are California, Connecticut, Delaware, Hawaii, Iowa, Kansas, Kentucky, Massachusetts, Michigan, Missouri, Nebraska, New Jersey, North Carolina, Oregon, Rhode Island, South Carolina, Texas and Utah.
** Most states limit where firearms are allowed and ban them outright from school property, prisons and jails, courthouses and other government buildings, and places where liquor is served.
A smaller number of states prohibit concealed weapons in places of worship, polling places, sports arenas, hospitals and mental health facilities and casinos.
A handful of states, including Oregon, place almost no limit on where concealed weapons may be carried.
** There is no federal ban on assault weapons -- semi-automatic firearms designed with military features to allow rapid and accurate spray firing. A ban adopted in 1994 expired in 2004 when Republican George W. Bush was president, and was not renewed.
** Seven states have imposed bans on assault weapons and the high-capacity magazines that increase their killing potential --- California, Connecticut, New York, New Jersey, Hawaii, Maryland and Massachusetts, plus the District of Columbia. Source: National Conference of State Legislatures, Law Center to Prevent Gun Violence, Brady Campaign and Brady Center to Prevent Gun Violence
(Compiled by James B. Kelleher; Editing by Greg McCune and Lisa Shumaker)