.... As long as the rifle was already producing good, tight groups, I have never been able to eliminate the "third-shot drift" (as I like to call it) - however small it may be. I have come to the conclusion that this is, in effect, a "natural occurance". My current rifles (BOTH of them) will drift the third shot approx. 3/8" to 1/2" away from the first two, usually to the left, no matter what I do.
My own rifles that do not exibit the so called "third shot drift" as you call it and I strongly disagree that this is a "natural occurance" with most rifles.