I often wonder if it is not so much a matter of breaking in the barrel as it is the whole rifle, getting the parts to work together, polishing the various surfaces that work with each other, etc. I am far from an experienced long range shooter, a few years ago I took one of my SIG SHR 970s-.270 Winchester-to sight in, the rifle was NIB, tightened the screws, first group at 100 yards about 1.5", next group 1.25". Perhaps advances in machine tools and techniques-CNC, e.g. allow more quality control oriented manufacturers
to achive finer tolerances, smoother surfaces, etc. Also different steels.