The shooter in a Texas church that killed 26 people Sunday used a Ruger AR-556, which is a variant of the AR-15 rifle — the most popular rifle in America.

Various versions of the AR-15 rifle have been used in some of America's most deadly mass shootings, including the Orlando nightclub shooting in 2016 that killed 49 people and the Las Vegas shooting last month that killed 58 people.

The AR-15 is trademarked by the Colt company, though other manufacturers market similar rifles under different brand names, like the AR-556 used by the church shooter. The 556 in the rifle's name is a reference to the 5.56 millimeter ammunition it uses.

AR-15s are sometimes called assault rifles, though this is incorrect because assault weapons are fully automatic machine guns and have been heavily regulated in the U.S. for more than 80 years.

The AR-15 was first developed in the 1950s and was eventually modified and adopted by the military and renamed the M16. The military version is fully automatic, meaning it will keep firing as long as the trigger is pressed. The AR-16 available for civilians is "semi-automatic," meaning the trigger must be pulled each time the weapon fires a round.

The guns are popular in America due to their reliability and their relatively low price tag. The Ruger AR-556, for example, has a retail price under $800. The guns can be outfitted with customized stocks, barrels, scopes and other accessories.

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