I have one aftermarket barrel (Brux) and more then a dozen factory barrels (Remington, Winchester, TC, Bergara, Browning, Ruger) My experience with break-in is that is will only make the rifle clean easier later in its life and resist getting dirty longer with a proper break-in. If the rifle is going to shoot well, you will know right away.
By this I mean, a barrel with no break-in will be accurate right away (clean) but may loose accuracy after as few as 20 shots. A rifle that has a break-in will be accurate for 100 shots or more. The aftermarket Brux barrel on my 7mm will shoot great for, well, I have never figured it out. I have put more then 150 rounds down it and finally just cleaned it because I wanted to. I have factory barrels that are approaching that number. I also have barrels that start grouping crappy at around 80 shots and need a cleaning. In particular a Kimber 84L in 270 starts to open badly at around 50 to 60.
I break-in a barrel according to what I see when I clean it. I usually take 1 shot and totally clean it using any powder solvent (Hoppes #9 or whatever) just wet a mop that comes in your cleaning kit and scrub back and forth. I don't use brass brushes anymore because I find that I don't need to. Then 2 wet patches and patch it dry. Then use a good copper solvent (like Butches Bore Shine). The thing about copper solvents is they use ammonia which turns copper blue. Wet the same mop and work it back and forth pushing a wet patch after about 2 minutes of scrubbing with a mop. Do this until there is no blue on your wet patch in between.
Then shoot 3 shots and repeat the cleaning. 90% of barrels are done breaking in after these 2 cleanings. Some may take 3 to 5 cleanings. The big thing is you will see that at some point the barrel will barely get copper fouled after your 3 shots and it will barely take anything to get the copper out of it. At that point the barrel is broken in. My worst rifles needed 5 cleanings to get broken in. My Brux needed 3 cleanings and both Bergara's needed 2 cleanings.
Good luck