This is after ~500 rounds. It looks different than some of the images I've seen in this thread. I'm new to guns... does it look like pitting? It's on the "roof" of the barrel, less than an inch from the feeding side.
P.s.: 9mm, 4.6" barrel. If the explanation given by Walther is accurate, I do not see this to be a problem, but these spots look like more than a few microns (millionths of a meter) thick (unless the underlying surface is a dull black color). To clean, I've only used Break-Free CLP (twice, sparingly), and Ballistol.
It appears there's minor cracking over the entire circumference.
Do yourself a favor before contacting Walther warranty...... get a brass bore brush for 9mm, wet the bore & brush with your CLP & vigorously clean the barrel bore. After 4 or 5 passes with the brush, run a patch through & you'll probably see black on it from what you missed cleaning. Keep repeating the process until the patch comes out clean after a run through with the brush. Then, take a look at the bore & it might be fine.
Whatever Walther uses in their barrels seems to make it harder ( for me, anyway ) to get them clean. My CZs & Sig clean up with much less work.