I've often read that American personnel in Vietnam loaded their 20 round magazines with only 18 rounds to improve reliability. Just how common a practice was this really? Were there really problems with the magazines that caused them to misfeed if fully loaded or was it incorrect reaction to the other problems the rifle was having at the time?
If the magazines were flawed, when did they fix the problem and start to phase out the older magazines? Was the same practice applied with the early (and very rare) 30 round magazines?