Quoted: Anyone ever stop to wonder how the gas tube is not supposed to ever need cleaning, due to the high pressure of the gas flowing through it, yet somehow, this high pressure stream turns filthy at the end of the tube, when entering the bolt carrier group? Like two inches further?
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Fluid dynamics.
In the gas tube, which is small, the gas retains high velocity and stays hot, blowing out anything that gets in, including whatever it might be carrying with it.
Once it gets into the expansion chamber inside the carrier, and inside the rest of the receiver, it expands, cools, and slows down, depositing all the carbon there.
Basically the same way a sound suppressor works. Or a fast, narrow river that widens out; the flow slows down and all the sand, rocks, etc. that were going along with it are now too heavy for the slower moving water to pick up, and gets dropped.
See pictures of the Mississippi River delta basin as an example.It's no real problem though, unless you like to do your examinations with white gloves and talk to your rifles.