Bart B., I will assume you are telling me the firing pin is the cause, the other option would be: Are you asking me?
I am a case former, I am a wildcatting case former, I make donuts, I have dies, methods and techniques that turn dies into donut making machines. I have never used a firing pin to make donuts, I have ask the question "Flow or stretch, flow and stretch etc.? and I have said I have never found skid marks on a case.
To your response, all I can say is "FANTASTIC!".