Quoted: I think you are misunderstanding how the AR15 works, and the role of the gas rings (if any) in a piston operated upper.
In a conventional AR15 the gas bled from the barrel feeds back through the gas tube, into the carrier behind the gas rings. There is nowhere for the gas to go, so pressure builds up, pushing on the gas rings (and hence the bolt) in one direction, keeping the bolt closed, and on the bolt carrier in the opposite direction, forcing it backwards. As the bolt carrier moves back it turns the bolt, unlocking it and eventually the bigger mass of the bolt carrier moving backwards, pulls the bolt open.
So in the conventional AR15 the gas rings do need to form some sort of gas seal -- but actually not a complete seal, just an impediment to gas flow, since there is a lot of gas under high pressure and a bit of a leak is not important.
With a piston upper the mechanism is completely different. Gas bleeds from the barrel and the gas tube takes it to a cylinder containing a piston - the gas pushes the piston back and a mechanical arrangement (push-rod) pushes the bolt carrier backwards. In this case, the gas rings are not performing any gas function -- basically, all they are doing is acting as a spacer/bearing for the rear end of the bolt.
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I am aware of how the AR15 gas system works. (Very good description though) The one thing that I would add is that as the Carrier continues moving back, holes in the side of the Carrier move past the gas rings and pressurized gas is allowed to escape out the ejection port.
I didn't say that gas rings were not needed, just that misalignment is not necessary. I suspect that the main function of the gas rings is to maximize the containment of debris inside the expansion chamber. This is where the above addition come into play. The gas has to go
somewhere. The rings simply contain the gas until the pressure has dropped enough to expel outside.
ETA: The gas
does push on the back of the bolt. This is part of an "equal but opposite" reaction similar to hydraulics. The bolt is locked in place at this time by the barrel extension so the gas cannot move it. The carrier is held only by a spring so it is free to move away (opposite) from the bolt. The bolt becomes, in effect, a fixed piston with the "cylinder" moving around it.
If the rings were not there, the main bearing surfaces of the bolt/ejector/extractor would all be exposed to greater fouling than they are now. I think this can be seen in the fact that gas rings are so cheap. If a tight gas seal was needed, gas rings would need much tighter tolerances, and they simply wouldn't be thin, cheap, stamped sheet metal parts. The gas rings on my AR float around on the bolt, and are not tight in any direction of movement, yet it has never failed to function.
IMHO, the reason for having 3 is redundancy, survivability and cost (see above). Redundancy is obvious, there are 3. If one fails, there are 2 more still working. Serviceability: small, thin parts are easier to replace in the field than is a single, large, thick part.
I also have a FAL so I am very familiar with gas piston actions as well. The fact that a piston powered carrier will cam/open the bolt and cycle the weapon shows that there is no need for gas pressure to act upon the rings in order for the weapon to function. This is perhaps what I should have stated in my first post.