Carbon from a gun will build up along the wings. They would always be cleaned at the end of the flying day. It was found that the carbon would cause corrosion and pitting on NMF aircraft. Even when missiles, rockets, or a gun is fired, the crew chief will always clean the area around the weapon being fired. The weapons troops would clean and lube the gun barrels at the end of the flying day. The rocket launchers would be cleaned and lubed. For a missile launcher the breach would be cleaned and in the case of a Sidewinder the launcher would be removed and cleaned in shop. Rocket pods would be removed and cleaned in shop. MER's, TER's and BRU's would be broken down and cleaned in shop.
After a specified amount of rounds being fired from a gun, it would be removed and totally cleaned and inspected. For the 20MM M-61A1 it was 100,000 rounds or one year which ever occured first.
No Crew Chief would ever allow carbon buildup to accumulate on his aircraft. That would be the fastest way I know to have the line chief all over him. He didn't want to spend the weekend on the wash rack washing aircraft for not keeping his jet clean.