Why is the AR-15 rifle .223 caliber so popular for those who perpetrate mass shooting? In addition to this latest church shooting where a AR-15 .223 was employed, this is weapon favored by many mass shooters such as these: Sandy Hook, San Bernardino , Orlando, Las Vegas, Umpqua, Ore., Arvada and Colorado Spgs. church shooting, Meridian, MS, and count less other mall shooting etc?