Washington (AFP) - Time and again, America's worst mass shootings have featured a common thread: the killer's use of a military-style assault rifle that is inexpensive, easy to use and deadly efficient.
The type of weapon, commonly known as the AR-15, is once again under scrutiny with critics calling for a ban after last week's massacre at a Florida high school that left 17 people dead.
- Why are AR-15s so lethal? -
The AR-15 is a semi-automatic weapon, meaning a user can fire off multiple shots in quick succession. Its cousin, the M-16, is a fully automatic version that has been used by the US military since Vietnam. Fully automatic weapons are banned for civilians.
AR-15s fire high-velocity .223-caliber bullets that are accurate over long distances and cause expansive, devastating wounds to soft tissue and internal organs.
These bullets -- which travel at triple the speed of a handgun round -- are popular among hunters for hitting targets up to a quarter of a mile (400 meters) away.
Stephen Paddock, the killer in last October's mass shooting in Las Vegas, had amassed an arsenal of weapons including assault rifles. He fired more than 1,100 rounds from a hotel suite into a crowd of concertgoers more than 400 yards (meters) away.
The toll was 58 dead and more than 800 injured.
- Cheap and easy -
Buying an AR-15 is easy. Depending on the state of residence, a prospective owner can walk into a gun shop and, after presenting a valid ID, buy a rifle or shotgun provided they can pass a federal background check.
This process looks at a buyer's criminal history or whether he has ever been committed to a mental institution. But even this basic check can be flouted in the case of private sales.
Nikolas Cruz, the man police say carried out last week's massacre at a Florida high school, had legally purchased the AR-15-type weapon used in the attack and had passed an FBI background check.
Aged just 18 at the time, the background check didn't raise any red flags. In many states, one only needs to be 18 to buy an assault rifle -- three years below the American age restriction on alcohol.
Many Americans can simply go online and order weapons for delivery. AR-15s vary in price but can be bought for as little as $500.
The National Rifle Association touts them for recreational target practice, hunting and home defense, although experts question their value for the latter two uses.
Part of the reason for assault weapons' popularity in America is that they are widely customizable, with owners able to add scopes, large-capacity magazines and a plethora of other accessories.
The Bureau of Alcohol, Tobacco, Firearms and Explosives doesn't know how many assault weapons there are in America -- they are prohibited by federal law from keeping a gun registry database.
According to the National Shooting Sports Foundation, the gun industry currently sells an estimated 1.3 million "modern sporting rifles and similar types of guns" each year.
Estimates vary, but there are thought to be as many guns as people in America, which has a population of more than 320 million.
- Bump stocks -
Vegas gunman Paddock increased the firing rate of the assault rifles he used with the addition of a "bump stock." Largely unheard of outside of America, these legal devices essentially turn a semi-automatic weapon into an automatic one.
The devices clip on to the back end of a rifle and harness the gun's recoil to bounce the weapon's trigger off the user's finger.
So, instead of a gunman having to squeeze the trigger repeatedly, the bump stock does the work for him, allowing him to empty a magazine in seconds.
President Donald Trump announced Tuesday he would support moves to ban the devices, though even that modest step is likely to be vociferous gun-rights advocates.