Mike, I do not clean - and have never cleaned my gas tubes. I just replace the gas tube when I replace the barrel. Currently, I have a gas tube with about 5k down it and no problems and I've personally owned one with 10k on it and when I removed it, it was just fine fouling wise - basically the only difference between it and a new one was that it was charred black all over.
I have read accounts of gas tubes becoming clogged through fouling. Most notably, it was a problem with the ammo used during the initial trials in the early 1960s. They discovered that a certain percentage of calcium carbonate in the powder would lead to fouling of the gas tube and eventual blockage.
I've also read online accounts of people claiming blocked gas tubes through fouling. In every one of those cases, they were using foreign-manufactured ammo, usually military surplus; but I've read one or two posts claiming the problem with Russian import ammo intended for the U.S. civilian market.
In my experience though, every time I've personally seen a clogged gas tube, it was inevitably a piece of pipe cleaner or Q-tip stuck in there from someone trying to clean it.
I know that port pressure at the gas block is around 12,000-15,000psi on a rifle and around 26,000-30,000psi on a carbine. My pet theory is that if 12,000psi of gas won't move it out of the gas tube, I'm not likely to have much luck with pipe cleaners and q-tips trying to get it clean. So I don't mess with it.
I do however put a drop of CLP in the gas key per the TM. I think this has less to do with cleaning the gas key than making sure the rings on the bolt are lubed though since that is usually where that drop ends up.
