As far as the stuff about AR15 barrels on the Bushmaster website goes, that is for chrome lined barrels only, and it says so in the text. I know because I'm the guy that wrote it. In our experience at the factory, barrels would get less fouling if they were fired 200 or so rounds before the first cleaning, then cleaned whenever the accuracy started to suffer. We recommended this on our lined barrels only, as the DCM rifles with unlined barrels we tried this on in testing didn't respond well to it. A "Foul Out" unit is probably the best bet for those, or any unlined barrel, and is handy for getting out fouling on lined barrels as well.
Everyone should be aware that this is one area that is being driven by popular demmand right now, with little or no real research being done by anyone in the industry. Barrel break-in is rapidly becoming a popular myth, where some people will tell you straight out that no rifle will shoot if it's not "broken in" right. Well, that's just crap. Any barrel made well will shoot well unless it's neglected or abused, and that's just a simple fact. Any effect breaking in a barrel may have would probably have to be measured with a micrometer at the target, as the difference would be almost neglegible.
I used to put it this way. Unless you are firing from a solid benchrest, with front and rear rests, handloaded match ammo made specifically for the barrel you are shooting, and firing single shots allowing the barrel to cool between each round, you won't be able to tell the difference anyway, so for crying out loud, go shoot your rifle! More time using it and less time playing with it, and you'll be a lot happier. Works on a lot of levels, if you get my meaning.
That's my $.02, if it's of any use to anyone. Don't go nuts with breaking in rifle barrels, it's not worth the trouble. As always, IMHO, YMMV, etc...
With my shield or on it...