I had a box stock Remington 700V in .308, that shot unbelievably well as it came. I swapped it off to my buddy for something I just had to have. (Ill never learn.
) He was convinced that fluting the barrel would make it even better. He had it done, and it was never the same again.
I have a suppressor for my AR's, one of which is an Armalite M15A4(T), that will shoot tiny little groups without the suppressor hanging off the end. With it on there, POI shifts a couple of inches both vertically and horizontally and the groups, while not bad, have never been as tight.