To be rather pedantic, the very first AR-15 rifles in the Vietnam theater worked very well. It was after the Army Ordnance Corps removed the chrome lining from the spec and changed the ammo from stick to ball powder that the rifles failed in the field. (the story is more complicated than that)
Skip to 46:30 for summary
https://www.youtube.com/watch?v=NYar4Zf8jH8