The Valentine's Day shootings at Marjory Stoneman Douglas High School riveted attention on the weapon used to kill 17 and injure 14 at the Broward County campus — the popular AR-15-style rifle.

Here are five things to know about the weapon.

1) What was the AR-15 designed for?

The weapon, named for original developer ArmaLite Rifle, was designed by legendary weapons maker Eugene Stoner based on his previous blueprint for an assault rifle to be used by the U.S. military. In 1958, the Army ordered the first tests on the rifle. It proved to be highly accurate; easy to switch among firing modes — single fire, semi-automatic and automatic; easy to control on full automatic; easy to reload, with a magazine inserted straight in and releasable with a trigger-finger button; and lighter for weighted-down troops to carry. It was not fielded for several years because it deviated from the pattern of earlier weapons, with its aluminum and plastic components and its development outside Army channels. The rifle first came into wide use as the iconic M-16 of the Vietnam War.

2) What makes the weapon different?

Stoner designed the AR-15 and its predecessor the AR-10 with his patented system to capture and channel gas released by fired cartridges to power the subsequent firing and reloading of the rifle. The AR-15 also has evolved into an open, modular gun platform that's easy to accessorize. With a few simple tools and no gunsmithing expertise, a user can modify the weapon or even assemble one from scratch with a variety of widely available parts. The AR-15 can also be adapted to fire a wide array of ammunition. Most of rifles, though, fire .223-caliber Remington or 5.56mm NATO ammo — each figure a measure of the bullet's diameter.

3) How many of the rifles are out there?

No one really knows. Gunmaker Colt bought rights to the design from ArmaLite in 1959 for a lump sum of $75,000 plus a 4.5 percent royalty on future production. Today, Colt's Manufacturing Co. says the platform is the most popular rifle in the United States. But the government does not track specific weapons platforms, according to the Bureau of Alcohol, Tobacco and Firearms. ATF's most recent data shows a total of 4.2 million rifles were manufactured in 2016 but provides no breakdown on types and models. The AR-15 became available to civilians shortly after it was developed for the military, but was never a big seller until after a mass shooting at a Stockton, Calif., elementary school on Jan. 17, 1989, that left five people dead and 29 wounded. "Before Stockton, most people didn't know you could buy those guns," Chris Bartocci, an author and former Colt employee, told CNN. AR-15 sales soared in popularity after a 10-year federal ban on assault weapons expired in 2004.

4) Are they used often in U.S. slayings?

Semiautomatic rifles like the AR-15 have been used in some of the country's deadliest shootings, such as those in Newtown, Orlando, San Bernardino, Las Vegas and the Valentine's Day shooting in Broward County. But overall, you're about 35 times more likely to be slain by a handgun than a rifle of any type, according to FBI statistics from 2014, the latest figures available. Of nearly 12,000 homicides nationwide that year, 8,100 were the result of firearms, and of those, 5,500 were caused by handguns and 248 were caused by rifles. Some 2,000 deaths were caused by undetermined types of firearms. Knives and other cutting instruments also proved more deadly than rifles, according to the FBI, causing nearly 1,600 deaths.

5) Is an AR-15-style rifle expensive?

As a class of weapon, the rifle costs far more than other types of rifles at the lower end. Depending on model and accessories, a new AR-15 can cost as little as $700 — about the cost of a 17-inch HP Notebook laptop computer. At the higher end, they run $1,700 and up.

Sources: Gun Digest, Colt, Time, Wired, Washington Post, AmmoForSale.com, FBI, ATF, WideOpenSpaces.com.