Soot on fire formed brass, but not on new brass- 7RM??
I reloaded new FL sized brass (7RM) working with Retumbo powder and the 175 SMK bullet I got great accuracy and a full gas seal with 70 grains of retumbo. I neck sized the once fired brass with the same load and bullet and when I shot these fire formed cartridges I was getting black soot on the entire neck and about 1/3 of the way down the shoulder. Curious as to why this happened with the formed and neck sized only brass and did not occur with the new-FL sized brass? All components, bullet, powder, charge, primer was the same?