Is that a brass neck diameter? If its a neck diameter the chamber is a little large in my opinion which would lead to the soot issue. I have an old Mauser military barrel that is large necked and it will actually split the necks if I dont keep them annealed. You should only have about .002 to .003 neck clearance from loaded to fired.Remeasured. Loaded is .2815. Fired is .2875. As I said in the first post, I backed off the sizing die one full turn. Do not think it is a head space problem. This was not a light load. I will have to fire the Winchester brass to see if this is different.