A while ago, I saw a thread on this on another forum. I think it was Gale McMillan (correct me, anyone if this isn't the case)who posted in it, saying that breaking-in a barrel doesn't add an advantage. The barrel doesn't know the difference between the amount of shots fired vs cleaning intervals, it's wishful thinking. That was the first time I saw anything de-bunking the break-in theory. I've never seen a magazine article saying it doesn't work, but I've seen articles saying the break-in process even restores accuracy of older guns that were never broken in when first purchased. When it comes to ultra long range cartridges, barrel erosion may occur within a couple hundred shots. Breaking in would only reduce the barrel's lifespan.
I have also heard that lapping barrels would cause the sharp endges of the rifling get rounded, causing decreased accuracy. If a barrel has been test-fired, there won't be any burrs to worry about, and any defects in a chrome-lined barrel are under the chrome.
With all that said, I don't know if break-in works, but wanted to present a rare opposing view I've once seen.