Soot on fire formed brass, but not on new brass- 7RM??

[Andy W]

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?

[keithcandler]

It is hard to believe how much a case can work harden on one firing, but it is true.

[Centre Punch]

Longdraw,
Are you using the Lee Collet die?
My buddy had the same thing happen when he used the collet die but not when he used other dies.
It turned out that the neck was not being adequately sized to hold the bullet tight enough in order to build up enough pressure to completely obturate the chamber.
Polishing a couple of thou off the mandrel cured the problem most of the time, my buddy has since changed dies.

Ian.