Is the direct impingement the only part of the rifle that is causing problems? I seem to have read stuff that said that is not the case. I thought I heard someone comment on sand sticking in around the locking lugs of the bolt. A gas piston only won't suddenly make the chamber shiny and clean.
That is another weakness of the system.
The AR-style bolt has (what, eight?) lugs on it. This has some advantages, it creates a very quick and short unlocking rotation which has ramifications all the way up and down the gas and fire control system. Remember that the original intent of the AR design was absolute lightest marching weight while still being well balanced with capacity-firepower and lethality. It also allows for even distribution of bolt thrust during firing, which means you can get away with much lighter parts, and facilitated the then revolutionary barrel extension chamber, which in turn allows for the aluminum non-stressed receiver that only need act as a guide-way for the moving parts.
OTOH, the multi-lugged bolt increased surface area and the complex topology increases exponentially the area for fouling and debris to gum up the action, and the leverage that fouling has to negatively impact cycling of the rifle.
One thing that occurs to me with the 'Just swap the uppers' philosophy of a more incremental design change to the AR, perhaps including a 6.8 or 6.5mm caliber change too, is that then the buffer tube is absolutely wasted space. A gas-piston upper can incorporate the recoil spring into the upper, which then allows for a true folding stock. The collapsing/telescoping stock the AR buffer tube originally forced as a compromise, is now well-liked because it allows for ergonomic adjustment to fit different statured soldiers, and account for the extra thickness of gear or body armor.
However, I see no problem making an adjustable AND folding stock.
