The AR15 absolutely is a MILITARY weapon- it was designed by gun engineer EJ Stoner for the military which adopted it as the M16 and added the automatic selector switch.

It became popular due to MARKETING by the gun manufacturers, especially once the patent expired and numerouos other gun manufacturers besides Colt could utilize and sell their own versions.

It was briefly banned by the 1994 assault weapon law but surged in sales after the law expired in 2004.