I've experienced the same thing in my 270x300RUM.
I would definitely say that it is directly related to high pressure.
Personaly, I've not loaded for the 300, but I would choose a powder with a slower burn rate than 7828.
Something more along the lines of Hodgdon Retumbo, H50BMG, US869, AA8700, or some surplus WC872.
Just a thought.
As for the marks, they are from the ejector pin hole. As the brass deforms under high pressure it flows back into the hole just slightly. When you open the bolt, it shears off what ever brass has flowed into the hole. Hence the shiny circle.