Don't know about the legality, but you're correct about the reality. There are number sliding and rotating parts in the lower receiver that determine whether it

Don't know about the legality, but you're correct about the reality. There are number sliding and rotating parts in the lower receiver that determine whether it fires single shot, burst or fully automatic. Modern M-16s have single shot and 3 round burst. The original 70s model had single shot and full auto.