The guy who does the test shooting for one of the major bullet companies (and a company that is reknowned for their accurate bullets) ran a 15,000 round test on moly a few years ago. He said that he would start seeing pressures spikes after about 50 rounds of molyed bullet went down the bore--and accuracy went south too. He said this was due to build-up in the throat that he could see with a bore-scope. A few more shots would knock the excess moly out, but until then accuracy was not so good.
I have run into some other thoughts on barrel break-in since this thread started:
Dan Lilja thinks a good layer of powder fouling HELPS REDUCE copper-fouling--and that's why we should clean the copper fouling out between the first shots, to allow the powder fouling to fill the little marks that even occur in lapped barrels.
Then other people will tell you that ALL fouling is bad, especially powder fouling.
