This is a phenomenon that I have always noticed as well.... no matter the rifle or cartridge used. I theorize that it has to do with barrel heating, in conjunction with barrel harmonics. Personally, I doubt that, IF the rifle you are using is producing good groups already (subject to your definition of "good groups"), that free-floating the barrel or glass-bedding the action, etc. will have any meaningful effect. I have personally tried free-floating the barrel in conjunction with bedding the action, switching ammo brands/types... and anything else that I could think of.... 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.