There are opinions both ways on whether the break-in does any good for a new barrel or not. I've done it on all 3 of my custom stainless steel barrels on AR15 match & service rifles. I use Shooter's Choice solvent to get powder fouling out and begin to dissolve copper fouling, followed by RemClean, an abrasive cleaner, to finish. After another patch wet with SC, I dry patch, then fire another round. After 5 single shots, I went to 3-shot groups for another 9 rounds. I then shot several 5-shot groups, cleaning completely after another 20 rounds. After that, I cleaned the bore completely after every 20 rounds a couple more times. Now, using moly'd bullets exclusively, I clean with two wet patches, then dry patch and oil. Every 300 rounds, I go in with RemClean again. My Douglas barreled Service Rifle has nearly 4600 rounds through it, and is starting to get rough enough to copper foul even with moly'd bullets. It still shoots good enough to clean the 200 & 300 rapid fire targets in a High Power match, but I'm beginning to doubt it's ability to hold the 10-ring at 600yds.
I guess what it boils down to is whether you believe all this helps or not. I'm sure it won't do much if anything for a chrome-lined barrel, such as the military M16 has, but it's worth the time and effort to me to get the most out of an expensive custom match barrel. If done correctly, with a rod guide to prevent damage to the chamber throat, I don't see what it can hurt.