Whether quite intending to or not, Jimmythebrain's post correctly characterized the alleged necessity for a "break-in" period as "propaganda."
Leave aside accurized target guns (they are a different breed of cat, built to perform to a different standard). If a production gun designed for service use will not reliably function straight of out the box with the ammo it was designed to use, somebody at the factory didn't do their job. The idea that it needs to be "broken in" is a rationalization for poor workmanship. What "breaking in" really means is to wear off the burrs that should have been tumbled, or stoned off by a fitter, or never raised in the first place if the cutter had been kept sharp, or to force parts to mate that are slightly misfitted because the tolerances were not maintained.
I have two VERY early P99s that were correctly made and fitted and neither of them needed to be pampered with any particular ammo. Walthers and other high-grade guns are test-fired at the factory. What kind of reliance could be placed on such a factory test under an assumption that there would be malfunctions that (maybe) would later go away? Why bother?
The caveat, of course, is that the ammo chosen by the purchaser ought to be reasonably similar to that used by the factory, which presumably knows what ammo the gun is designed for. I don't know what Walther uses today, but nothing in their manuals or any factory specifications that I have come across suggests that any standard 115 or 125 gr. load should be regarded as unreliable.
M