I'd assumed it was a given that the airflow around the guns, of every type, would have carried tiny fragments of metal chipped off the projectile during firing. As with engines, this means that over time, this build up of tiny, oxidising and oxidised flakes of metal would become visible. Mixture of browns and greys (maybe very slight green - brass?), I suspect, around the guns, and a more oily (gloss black) mix, but still containing rusting flakes of engine, around the exhausts.
But the $64,000 question is, how much time? Aircraft had a strict regime of servicing intervals, which started at around 24 hours flying time. In combat mode, a Spitfire's endurance was about 45 minutes, 2 hours at cruise, and they didn't fire their guns on every sortie. Even if they did, it was the armourer's job to "pull through" (i.e. clean and lubricate) every barrel, as well as replenishing the ammunition. Any small fragments, caught in the rifling, would have been removed by the pull-through cloths (2" x 4" if my memory's correct,) and there were usually three per gun, first to clean, second to lubricate, third to remove any excess oil from no.2.
He also had to fit the covers on the guns (or leading edges mid-BoB,) and any attendant residue would not have helped, so should have been wiped away from inside the gun tunnels. According to an ICI inspector it was common practice to wipe surfaces with petrol-soaked cloths, and there isn't much that can stand up to that sort of treatment.
If (big "if") an airframe saw combat, and fired its guns, using up all of its 15 seconds-worth of ammo, it might go in for a "24 hour check" after only 32 sorties, which, at the height of the Battle, could be 8-10 days (assuming that the aircraft suffered no damage, or a frightened pilot didn't pull so many Gs that he wrinkled the wings' surfaces by more 1/10", in which case the wings had to be replaced.) It's very unlikely that an airframe would have left a servicing bay in the same condition in which it entered; "Chiefy" would have had a fit.
Flakes of engine, in the exhaust staining, is something I've never heard of. It was more usual for wear to be confined to items like piston rings, or gudgeon pins, in which case they ended up in the lubricating oil, then the oil filter, which was regularly inspected for "extras." It was just such an eventuality that lead to K5054 needing an engine change very early in its flying programme.
Edited by Edgar, 05 April 2012 - 08:19 PM.