I can finally weigh in on this topic... for what little that's worth.
I have two new rifles. Santa Claus (that would be me) was kind this year. I... I mean Santa... brought me a Remington VTR in .308 and because the stock was horrific (his words not mine... I would have cussed) he also brought a nice HS Precision stock for it. A couple of weeks later he brought a Sendero SF II in 7mm Rem Mag. Remarkably it came with a good stock (HS Precision).
I happily loaded 50 rounds for the VTR and ran off to the range just as soon as I got the scope mounted and bore sighted (The Sendero didn't have eyes until yesterday so it stayed in it's crib). After 40 rounds the rifle had degraded to 3" - 4" groups (group is a term that I am using loosely) at 100 yards. Not exactly what I was hoping for. It would kill a deer but I had 0 confidence in the rifle at that point so it was looking like I was going to leave it home this season. I was cleaning between 5 but no copper remover. Just Shooters Choice and a couple of patches. I was there to shoot not clean
Needless to say when I got home I did a search for barrel break-in and found 5 or 6 disagreement sessions
with tons of conflicting advice.
I finally picked the shoot 1 / dry bore snake / copper remove / nylon brush (more controversy) / dry patch until patches are clean / repeat method (on a side note- have barrels become so crappy that they can't take a brush? Jeez man... I brushed the hell out of all of my rifles, with a bronze brush and Hoppes #9 solvent no less, when I was a kid and they all shoot as good as they ever did... which was good enough to kill a deer out to 200 yards. I would gladly do away with the brush step. It's a bunch of extra work... I'm just curious).
I paid for factory ammo
(very painful. I may be traumatized for life) and ran 20 rounds through it and my Sendero using the above "break-in" method. I was at the range all day "breaking in two rifles. It was awesome
The Sendero settled down by about round 8. The VTR wasn't settled down until the last 6 rounds. It's consistently grouping inside an inch at a 100 yards now and that's with over priced, sloppy ammo. I'm hoping to begin the process of finding it's sweet spot on Friday and bring it in a little and playing on the 200 range. I've got a batch of .010, .050, .090. and .130 off the lands loaded (both calibers). Just for fun I'm going to reload the factory brass that I shot today and take it back too. Friday should be a good day.
So- does the break-in method work? I think that it helps. The rifles cleaned easier as I progressed through the steps. Towards the end I shot 3 rounds and then cleaned. First the dry bore snake and then I ran a wet (Montana Extreme Copper Killer) copper cleaner patch through it. Waited about 5 min. Dry patch. There was no telltale copper residue btw. Brushed with Shooters Choice (probably will stop this just to see if I can do as well with just patches but I find it hard to believe a nylon or even bronze brush would hurt a barrel). Then 3 dry patches. The first one was dirty... the second one was clean. The 3rd one was snow white. Compare that to 6 - 8 patches before I got a clean one and never snow white when I started the "break-in"
Caveat- These were both Remington factory rifles. No tricky stuff. I will follow the barrel / rifle builders recommendation if / when I buy a custom. JMHO