Predecessors M16 rifle

colt s first 2 models produced after acquisition of rifle armalite 601 , 602, , these rifles in many ways clones of original armalite rifle (in fact, these rifles found stamped colt armalite ar-15, property of u.s. government caliber .223, no reference them being m16s). 601 , 602 identified flat lower receivers without raised surfaces around magazine , green or brown furniture. 601 adopted first of of rifles usaf, , supplemented xm16 (colt model 602) , later m16 (colt model 604) improvements made. there limited purchase of 602s, , number of both of these rifles found way number of special operations units operating in south east asia, notably u.s. navy seals. major difference between 601 , 602 switch original 1:14-inch rifling twist more common 1:12-inch twist. these weapons equipped triangular charging handle , bolt hold open device lacked raised lower engagement surface. bolt hold open device had slanted , serrated surface had engaged bare thumb, index finger, or thumb nail because of lack of surface.

the united states air force continued use armalite ar-15 marked rifles in various configurations 1990s.