I offer no scientific supporting evidence, just my observations. Brand new Sako Quad in .17hmr, shot exceptionally good from the first shot. Brand new Sako .308 took maybe a box of 20 before I was getting good groups. Re-barrelled Sako 22-250 took about 100 rounds before I started to achieve small groups.
I've loosely followed barrel break in cleaning regime in some rifles, where in the past I haven't & have noticed no long term difference in accuracy between the 2 methods.