I have never seen any evidence, documented OR anecdotal, that shows breaking in a barrel has any meaningful bearing on the life of the barrel, overall accuracy or anything else. The one exception I will make to that statement is I believe breaking in a barrel DOES result in the gun being generally easier to clean, with each subsequent trip to the range.
I make this claim based on the following experience. A friend of mine bought a Remington custom shop Model 700 in .308, with either a Hart or McMillan 20" bull barrel (can't remember which). He installed a Leupold 10X scope, a Harris bipod and had the trigger adjusted down to about 2.5lbs. He brought 80 rounds of Federal Premium ammo with 168gr SMK bullets. We basically followed the process that Stretch outlined above: Fire one, clean...fire 3, clean...fire 5, clean. At first, it took a whole lotta cleanin', to get the patches to come out clean. I was pretty much astounded to find that after the last rounds were fired, in 5-round groups, it was much easier to get a clean bore than it was when we started. (By the way, the 100 yard group, for 80 rounds was...one hole and it was just barely over 1.5 inches!)
Now, being the suspicious sort, I immediately tried to figure out why his gun barrel had gotten so much easier to clean, as a result of that process. I talked to my old buddy, Al, down at the range. (Al had been a bench rest shooter in the 60's and could still put 3 in an inch, even though he was well into his 60's, himself.) Al listened to my story, nodding his head and getting that quiet grin he always had whenever I asked a question that he could expound on. He explained it like this, and whether or not it's true, I took it as gospel and, later, proved it to myself. He said that all guns come with very minute machine and tool marks in the grooves and lands of the barrel. By breaking in a barrel he explained that those loads, and the subsequent scrubbing with the solvent and brass brush, helped smooth out those marks. Knowing a bit about the relative hardness of copper bullet jackets and brass, I felt this was a bit of a stretch, but he told me to take any of my rifles and pretend they were brand new. Shoot them as though I was breaking in the barrel...and see if they didn't clean easier after 50-100 rounds, than they did before.
I'll be danged if it didn't work, just like he said it would! Not only on my relatively new 270, but also on a (then) 20 year old 25/06. He pointed out that most folks hardly shoot their rifles, with a clean bore, enough to see this happen, and when I got to thinking about it, that made sense to me. He claimed that a guy would buy a new gun, shot 15-20 rounds through it, clean it, then repeat that 2-3 times a year. What he was trying to get through to my young and thick skull, was that firing a round in a CLEAN BARREL, is what it took to smooth out those tooling marks.
Now, I'm not going to try to convince anyone else that it works, but in my highly subjective opinion, it does! Also, I will offer the disclaimer that manufacturing processes have improved quite a bit and that the barrels on guns made in recent years may not benefit as much. However, that .308, which was purchased and broken in about 1990, surely did benefit, at least where cleaning was concerned.
