REVEALED: Suspected Orlando gunman used .223 caliber AR assault rifle, 9mm semi-automatic pistol

New York: Omar Mateen, the 29-year-old man suspected of killing 50 people in a Orlando nightclub early on Sunday, used Amerrica's most favourite assault rifle - AR-15, said investigators.

According to the federal investigators, the AR -15 asasult rifle was used in the 2015 San Bernardino shootings, the 2012 shooting in a movie theatre in Aurora, 0he 2012 Sandy Hook elementary school shooting besides Orlando.

The AR-15 is the civilian version of the M-16 assault rifle and a reliable estimate suggests there are around 5 million of these weapons in the US, making it America’s most popular rifle.

Guns traced in #Orlando shooting: .223 caliber AR type rifle and 9mm semiautomatic pistol. Similar to those pictured pic.twitter.com/Ao5pcLtwBg

— ATF HQ (@ATFHQ) June 12, 2016

While the weapon has rarely been used in crimes such as robberies, it has been linked to some of the worst mass shootings in recent years, said a Hinduistan Times report.

Omar Mateen used an AR-15, which can pump out 45 rounds a minute in semi-automatic mode, in the attack on the Pulse nightclub in Orlando, Florida. The US state only requires a person to fill out an identification form and pay $8 for a background check to buy a deadly weapon such as the AR-15.

The second amendment to the US Constitution states that the right of American citizens to “keep and bear arms” shall not be infringed and organisations such as the National Rifle Association have zealously defended this right despite growing opposition to firearms following a series of mass shootings.

The AR-15 was first built by the firm ArmaLite in 1959. It sold the design to Colt, which first began selling civilian versions of the rifle. Colt’s patent expired long ago and versions of the AR-15 are now made and sold by several firms, including Bushmaster, Remington Arms and Smith & Wesson.

Versions of the AR-15 are available in the US for prices ranging from $499 to $899. Some with additional features and a higher calibre sell for more.

What makes the AR-15 particularly deadly is that it can be illegally modified to fire in full automatic mode – as was done by Syed Rizwan Farooq and Tashfeen Malik, the shooters who killed 14 people in San Bernardino last December.