About this detail of the Tiger
The barrel was in two parts; an inner sleeve (Seelenrohr) and an outer jacket (Mantelrohr). These are both modelled here. Notice that a ring was screwed on to hold them together. I am guessing that they were painted in primer during manufacture; there exist photographs of tanks that seem to have replacement barrels painted in a dark colour.
[1] Drawings 5155-2 and 5155-7, Bundesarchiv