Any of various automatic or semiautomatic rifles designed for individual use in combat.
A military style automatic rifle or carbine that fires a shortened rifle caliber round or lower power smaller calibre round larger than pistol ammunition from a high capacity magazine.
From the Meriam Webster Dictionary
any of various intermediate-range, magazine-fed military rifles (such as the AK-47) that can be set for automatic or semiautomatic fire also : a rifle that resembles a military assault rifle but is designed to allow only semiautomatic fire