Any barrel that truly needs breaking in is not properly finished. The difference between a barrel that is "seasoned" by a few very slow shots and extra-heavy, obsessively frequent cleaning and one that is "just shot" in an AR-15 is lost in the variability of the rest, the shooter, the optic, and the conditions. There is, however, a major problem in demonstrating this definitively: no two barrels, no matter how carefully matched they are in production, will respond identically. They'll be close, but they won't be identical. So there is no real way to do a scientifically controlled test with one barrel "broken in" and another "just shot" that has any real validity for comparison.
A brand new GI rifle (which will of course has a brand new chrome lined barrel) will go right onto the rack, and the rifle will be fired. No break in is required.