Kopel says the weapons, which are modeled after the M-16 military rifle, are also highly customizable to each person.
"You can add or change a scope or barrel of your choice," Kopel said.
The AR-15 also makes it easy to shoot a lot of ammunition. Owners can purchase magazines that carry from 20- to 60-round magazines.
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While semi-automatic rifles have been around since the turn of the 19th century, they became more popular during the Vietnam Era when returning soldiers wanted a civilian weapon that resembled the M-16 rifle they used while in the army. The main difference was that they the fully-automatic function was prohibited.
Congress banned many types of assault rifles from 1994 to 2004, but the law failed to cover all semi-automatic weapons. Rather, certain barrels, stocks, grips and other features were restricted, according to Kopel.
"You had guns that were compliant with the law," Kopel said. "But the gun internally was the same" as before the ban.
The law did not apply retroactively to assault weapons that had already been manufactured.
"We do not know when the assault weapons involved in the shootings in San Bernardino were manufactured, nor how they ended up in the hands of the shooters after having been purchased legally two years ago by someone else," said Randolph Roth, a history professor at Ohio State University who has written about the history of gun violence. "But if the weapons were manufactured after 2004, it would mean that the lifting of the assault weapon ban was responsible for those particular weapons being in circulation."
Roth notes that while the number of guns manufactured in the United States has skyrocketed from just under 3 million in 2001 to nearly 11 million in 2014 (according to ATF figures), the number of gun owners has been declining (according to the General Social Survey).
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Gun ownership is also highly partisan, with ownership by registered Democrats less than half that of registered Republicans.
Efforts to regulate the sale of assault weapons, also known as "sport rifles" or "tactical rifles," are pretty much dead, according to Ladd Everett, spokesman for the Center to Prevent Gun Violence.
Everett says that the Republican party has blocked any attempts at gun control laws, including universal background checks, greater sharing of mental health data, and restrictions on certain individuals to obtain weapons. There hasn't been a huge push on the Hill because our side has never had the votes," Everett said.
"We've had some real dysfunctional congresses, even in the realm of the gun issue. It's an issue that is red hot, so much of their (the National Rifle Association's) messaging is around bans."
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For Everett, the focus has been on attempting to get universal background checks and research funds for gun violence studies. Regulating assault weapons isn't in the cards.
"When will we get back to this?" Everett said. "I don't have a clear answer."
In contrast to gun violence in the United States, since the mid-1990s, Australia's firearm mortality rate has dropped from 2.6 per 100,000 people to just under 1 per 100,000, according to the Australian Bureau of Statistics. The rate in the United States is more than 10 per 100,000, according to the U.S National Vital Statistics Report, according to the Post.
In the 18 years leading up to the Port Arthur massacre, there were 13 mass killings in Australia, which instituted a massive gun buyback program afterward. There have been zero in the 19 years since.