It can be a problem after many (thousands) of rounds. Pipe cleaner is about the best you can do for field cleaning. You could drop the entire upper into a solvent bath if you are back home from the range. I have put several thousand rounds through several gas system AR’s and have yet to have to take one apart to clean the gas system or even have to solvent bath one. But it could come to that if it gets dirty enough.
“What keeps the gas tube clean?
The cleaning instructions does not seem to mention it.
Or is it not a problem?” [Bitwielder1, post 10]
The system was originally designed to function with a specific load of a specific propellant, propelling a specific bullet to specific velocity. It’s pretty much the same with any self-loading firearm. And it’s the way the military has always required small arms of this type to work.
In practice, this means the gun will operate reliably when the ammunition uses only a relatively narrow range of propellant types, and bullet weights/types. Generally fine for military purposes, but when someone (often civilian users) changes propellant types or loadings, or bullets, they run into trouble quickly.
Early military users of the M16 ran into problems when the contractor loading ammunition substituted ball powder for the chopped-tube powder that rifle was originally designed to work with. Ball powder is produced in ways entirely different from the chopped-tube sort, with different additives. Function was greatly affected, fouling went way up, and gas tubes were clogged by residue from a drying and pH-balancing agent used as an additive. The technical details were reported many years ago in the print edition of American Rifleman magazine.
In most situations, the AR-15 gas tube is kept clean by the high temperatures & pressures of the propellant gases going through it. Carbon and other components of fouling normally stay in a gaseous state and are vented out of the rifle before they precipitate out and stick to parts.
The gas system of the M1 Carbine met similar design criteria: it was decided that disassembly and cleaning would be impractical for the individual soldier in the field, so the gas port was moved farther back on the barrel, thus utilizing propellant gases that were much hotter and at higher pressures, minimizing deposition of fouling. Since users could not clean the Carbine’s gas system in detail, the War Dept also required that companies making 30 Carbine ammunition use non-corrosive priming. It became the first US military cartridge to be thus primed. Standard 30-06 rifle ammunition was loaded with corrosive primers all through WW2 and into the early 1950s.