I'd bet all my paycheck for a year that if you take 2 identical Savage 10's, and break them in McMillan style (no breakin), and then the traditional 1-shot/clean method, you'll notice a profound difference. I've done it with 2 Ruger 77/22's and I can show you the difference. It's there, no question, no doubt. 100%, it's there.
What is the difference? As I've said 10 times on this forum (and no one seems to pick up on or debate, or they just ignore) it's not necessarily a question of accuracy. Ask the benchrest guys why they do it on factory barrels.
It helps to seal the microscopic pores in the metal, so I'm told, but what I know is it makes the gun easier to clean, and less prone to fouling....which has a beneficial byproduct in that you can stay accurate for a longer period of time (hence the Benchrester's fondness for this procedure). If this method was BS (as McMillan states), then the top benchresters in the world would have surely found this to be the case - yet they break their barrels in PRECISELY because of what I'm saying. It keeps a barrel from fouling and makes cleaning easier.
I've done it both ways and stick with the break-in on the guns I rely on for very good accuracy (below .5 MOA). Combat or plinking guns (or those with chrome lined barrels) need no such treatment.
McMillan was only challenging ONE facet of the claim, and can be said to be partially correct but even a GOD such as he can be wrong or narrow minded.
Put it this way, does it hurt anything? Why not give it a shot and see if I'm not right. I didn't believe in it at first either (sounded like voodoo) but I know from my own personal experience, which trumps blindly following McMillan anyday.