No Powder & a Crud Ring

So when i was testing out the Winchester W209 primers in the Wolf, I mainly wanted to see how bright and how long the flame was that came out the end of the barrel just by using a primer. After popping 6 primers i went to run a patch down the bore to remove the fouling and was shocked to see a crud ring forming 2" in front of the Breech plug just by using primers.

That was interesting as i never would have thought that just a primer could start a crud ring. And WHY only in that spot so consistently?

Zero oil of any kind in the bore during this test.