since i got out of the service i will sometimes use pipe cleaners to clean out the gas tube. we had some dirty ball ammo to run through the m16s back in the day but it wasn't near as dirty as the russian & yugo corrosive stuff i am running through the 7.62x39 uppe rand that 5.45x39 wolf is also super nasty as is 7n corrosive. i generally clean out the tube after a tin of 1080 rounds-- i could probably go much longer two or three tins maybe but i don't shoot that much at one time and i might forget to clean it out so i only go one.
back in the day i don't recall cleaning gas tubes at all. the gas tube was cleaned by the unit armorer mostly. he would take them off the weapons number them and soak them in some kind of solution overnight then a detail the next day would clean them out with compressed air and pipe cleaners. IIRC.