Okay, if one considers a thorough cleaning and a bit of bore polishing a break-in, I've done that. The "break-in" I was referring to was a subject of great interest a while back, where people were shooting new rifles, for example, 5 shots, clean, 5 shots, clean, repeat 4 times, then 10 shots, clean, repeat 5 times. That's what I considered a "break-in".
I really don't think there are many shooters who would literally just go out and shoot a new rifle without a thorough cleaning first....even in this world of fast food and instant gratification.
I havbe a book on building muzzleloaders where the author recommends scrubbing the bor with steel wool on a jag, for, IIRC, "about a hundred hundred times, or three songs on the radio."
There is also a company in the US that sells bullets coated with a fine abrasive, the idea being to fire these down the bore to polish it. I've only read one review of the system (on Chuck Hawks' website), the reviewer seemed happy with his improvement.