When you break in a barrel, what you're essentially doing is a final lapping/burnishing with bullets. Most BR match barrels were lapped as a part of the manufacturing process. Factory barrels likely were not. What this means is that it takes more bullets to lap/burnish. All this works better & faster if the bore is reasonably free of copper when you fire (polishing the copper fouling does nothing useful). And while you want just a little powder/primer residue in the bore (it is a mild abrasive), you don't want to build up carbon. Carbon fouling is by far harder to remove & more damaging to the bore.
The best way to learn about all this is to get a borescope & check, especially if you have a factory barrel from a manufacturer who has been known to make rough barrels -- good, but rough. I know, borescopes are expensive. Lacking one, I'd use the modified BR formula -- a couple of rounds & clean, etc, until the copper fouling diminishes. If you don't allow carbon fouling, the only variable is how long it will take. Remember, more damage can be done quickly by bad cleaning techniques. You might find that it takes 100 or more rounds before the barrel is as accurate as it will get, but of course, you won't do "five & clean" for 100 rounds. In breaking in a barrel, the only real damage you can do is bad cleaning; less than ideal "break-in" technique just means it takes longer.
