The Second Amendment has been placed in both media and political spotlights following last month's elementary school shooting in Newtown, Conn., and Friday's high school shooting in Taft, Calif.

According to the Congressional Research Service, in 2009, there were an estimated 310 million firearms in the United States (not including military-use weapons). Of those, 114 million were handguns, 110 million were rifles and 86 million were shotguns. That number reflects 98 guns for every 100 citizens.

A separate calculation done in 2010 by the Government Accountability Office, estimated that 118 million handguns were available for sale to, or possessed by, United States civilians.

Still, a large majority of citizens don't fully understand the laws or know their rights to carry them.

U.S. gun laws control the sale, possession, and use of firearms and ammunition. State laws sometimes vary, and are independent of existing federal firearms laws, although they are occasionally broader or more limited in scope than the federal laws. For instance, some U.S. states have created assault weapon bans that are similar to the expired federal assault weapons ban.

State level laws differ significantly in their content, form and level of restriction. Forty-four states have a provision in their state constitutions similar to the Second Amendment to the U.S. Constitution. The exceptions are California, Iowa, Maryland, Minnesota, New Jersey and New York. However, in New York, the statutory civil rights laws contain a provision virtually identical to the Second Amendment. As well, the Supreme Court of the United States has held that the protections of the Second Amendment apply against state governments, as well as their political subdivisions.

Firearm owners are subject to the firearm laws of the state they are in, and not exclusively to their state of residence. Reciprocity between states exists in certain situations, such as with regard to "concealed carry" permits. These are nationally recognized on a state-by-state basis. Some states do not recognize out-of-state permits to carry a firearm at all, so it is important to understand the laws of each state when traveling with a firearm. In 1995, the Pennsylvania General Assembly gave the attorney general the authority to enter into reciprocity agreements with other states. The Commonwealth of Pennsylvania currently has formal written reciprocity agreements with eighteen states that recognize a valid license to carry firearms issued by Pennsylvania. Conversely, Pennsylvania recognizes valid firearm carry licenses/permits from those states.

In some cases, state firearms laws can be considerably less restrictive than federal firearms laws. Although, this does not confer any de jure immunity against prosecution for violations of the federal laws. However, state and local police departments are not legally obligated to enforce federal gun law as per a U.S. Supreme Court's ruling.

Now, rising calls for state and federal bans on assault weapons and high-volume ammo clips have resulted in a large increase in gun sales. Some states and localities place additional restrictions on certain semi-automatic firearms that they've defined as "assault weapons," or on magazines that can hold more than a determined number of rounds of ammunition.

Some states and localities also place additional restrictions on NFA weapons. NFA weapons are weapons that are heavily restricted at a federal level by the National Firearms Act of 1934. These include automatic firearms (such as machine guns), short-barreled shotguns and short-barreled rifles.

The Federal Assault Weapons Ban (AWB), or Public Safety and Recreational Firearms Use Protection Act, was a subtitle of the Violent Crime Control and Law Enforcement Act of 1994, a federal law that included a prohibition on the manufacture for civilian use of certain semi-automatic firearms, so called "assault weapons." The 10-year ban was passed by Congress and signed into law by President Bill Clinton in September 1994. The ban only applied to weapons manufactured after the date of the ban's enactment. The Federal Assault Weapons Ban expired on Sept. 13, 2004, as part of the law's sunset provision. There have been multiple attempts to renew the ban, but no bill has reached the House floor for a vote.

It's impossible to know for certain how many guns are in private hands, because there is no central firearms registry. The 1986 McClure-Volkmer Act forbids the federal government from establishing any "system of registration of firearms, firearm owners, or firearms transactions or distribution." And the 1993 Brady Act prohibits the establishment of any electronic registry of firearms, firearms owners or firearms transactions.

According to the annual statistical report from the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, in 2010, more than 5.4 million firearms were manufactured in the United States.

According to state data compiled by the GAO, there were approximately 8 million active concealed-carry permits in the United States as of the end of 2011.