PHOENIX (AP) — Military-style semiautomatic rifles have been used in at least four high-profile shootings in the past year. Bushmaster variations of the weapon have been used in two recent attacks, including the Newtown, Conn., school shootings and the Christmas Eve ambush slayings of two New York firefighters. While commonly called the AR-15, Colt and Armalite have been the only companies to make weapons by that specific name. Many other manufacturers, however, now sell similar versions of the rifle largely styled after the military's fully automatic M-16.

A look at the guns, their history and why they're so popular:

HOW WERE THE WEAPONS DEVELOPED?

Armalite first built the so-called AR-15 rifle for military use, but the design was later acquired by Colt, which produced the M-16 automatic weapon for the U.S. military. In the early 1960s, Colt then began marketing the semiautomatic AR-15 rifle largely as the civilian version of the fully-auto M-16. Many other companies have since begun manufacturing and selling AR-15-type rifles, but under different names, including the Remington Arms R-15, Bushmaster X-15 and Carbon 15 and the Smith & Wesson M&P15. The AR-15 has become the commonly-used generic term for all similar rifles.

WHAT ARE THEY USED FOR?

The AR-15-type rifles and .223 caliber ammunition are largely used for hunting small game like coyotes and prairie dogs. They also are extremely popular in shooting competitions due to the light weight of the gun and ammunition and the weapon's accuracy.

HOW DO YOU PURCHASE ONE?

Most military-style semiautomatic rifles were restricted for sale under the 1994 assault weapons ban that expired in 2004. Today, such weapons can be purchased in gun stores across the country.

ARE THERE ANY LIMITATIONS ON SALES?

Licensed dealers must first run a background check on the buyer to determine whether they are eligible under state and federal laws to own the weapon; convicted felons, for instance, cannot legally own firearms. However, private sellers of such rifles are not required to perform background checks. This would include a person selling their private collection to a buyer from their home, as well as private sellers hawking their weapons at any number of dozens of gun shows that occur nationwide every year.