The type of weapon, commonly known as the AR-15, is once again under scrutiny after a gunman used the rifle to kill 19 children and two teachers in Uvalde.

Here are some key facts about the AR-15:

- Why are AR-15s so lethal? -

The AR-15 is a semi-automatic weapon, meaning a user can fire multiple shots in quick succession.

Its cousin, the M-16, has been used by the US military since Vietnam. While some military assault rifles are fully automatic, civilians are prohibited from owning such weapons in most circumstances.

AR-15s fire high-velocity bullets that travel at triple the speed of a handgun round, are accurate over long distances, and cause expansive, devastating wounds to soft tissue and internal organs.

Salvador Ramos used a high-end AR-15 from manufacturer Daniel Defense to attack Robb Elementary school on Tuesday.

While handguns account for more deaths per year in the United States, AR-15s are frequently used in high-profile mass shootings.

- 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 they have ever been committed to a mental institution. But even this cursory check can be flouted in the case of private sales.

Ramos legally purchased two AR-15s and several hundred rounds of ammunition shortly after turning 18.

The National Rifle Association (NRA) touts the rifles for recreational target practice and home defense, but critics say their lethality means they do not belong in civilian hands.

Part of the reason for the popularity of AR-15s 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 does not 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, more than 16 million of the rifles had been sold to the American public by 2018.

Assault weapons were banned in 1994 under president Bill Clinton, but the restriction lapsed in 2004 amid pressure from the powerful NRA, and congressional efforts to renew the prohibition since then have failed.