The P-47's blast tubes were made (I believe) of aluminum. They were also one of the items that was undoubtedly subjected to some pretty loving maintenance - probably at least wiped down after every flight, ends retaped, and the entire tubes removed whenever the guns were cleaned. They also contained most of the muzzle flash, so, like the Brits' Hispano 20mm cannons, you don't see much if any staining on them.
(snip)
Even on a wartime aircraft, the metal is clean and shiny.