The Colorado suspect allegedly used an AR-15-style pistol. Here's how it differs from an AR-15-style rifle

The 21-year-old suspect in Monday's massacre at a Boulder, Colorado, supermarket allegedly used a Ruger AR-556 pistol -- what one expert called "a semiautomatic made for combat" that is easy to carry and has "the firepower of a long gun."

source https://www.cnn.com/2021/03/24/us/boulder-colorado-weapon-ruger-ar-556/index.html