The three round burst feature was adopted because in Vietnam-and other conflicts-it was found troops issued selective fire weapons kept them set on full auto as a matter of course. Plus the close ranges, heavy foliage and jungle cover, and frequent night actions in Vietnam made aimed fire rather ineffective. And there is a difference between what recruits in Basic do under the stern and watchful eye of their DI as opposed to a combat situation where the action comes fast and furious-and unexpected
and everyone is hollering and cussing, the officers are trying to determine where the threat is coming from, the NCOs are trying to get their men off the ground to return fire, etc.