AR-15
A broad term for a sporting and military rifle originally developed by Eugene Stoner while working for the ArmaLite firearms company, a division of Fairchild Aircraft (The patent was later sold to Colt because of financial issues). The original design (released in 1956), was based on the AR-10 in .308, was revolutionary at the time for using a .22 caliber intermediate cartridge, and a semi-direct-impingement . This cartridge offered low recoil and similar terminal ballistics to the larger .308 Winchester (7.62x51mm NATO). As for the idea that the AR-15 is a dangerous weapon of war designed to kill, and that actually holds some truth. Eugene Stoner originally designed the AR platform for military contracts. However, Stoner designed the AR not for pure killing, but for these traits: Lightness, low recoil, modularity, and ammunition size while still retaining enough wounding potential to get the job done. As for the reason that they are used in mass shootings? First off, most people in America do not know much about guns and are quick to assume that any black "tactical" rifle is an AR (Even if they're not). Secondly? They're the most popular rifle in America. For the same reasons I just listed, the AR is extremely popular here, and is readily available. It's not something to be scared of! Go shoot one, and you might see that.
Eugene Stoner designed the AR-15 primarily for it's weight and not for it's wounding potential.
By Xxx_Eug3n3_St0n3r_xxX