The front of an AR bolt has gas-rings to reduce blow-by into the front of the bolt carrier.  But this only seals half of the expansion chamber in the bolt carrier.  Does anybody know why the original design did not include gas rings on the bolt stem also?  Seems like this would reduce the fouling around the firing pin.  I am also wondering if most of the fouling that is evident on the front of the bolt after firing is the result of gas leakage around the gas rings on the bolt, or is it due to leakage between the gas tube and the carrier key?

I’ve kinda wondered that myself.

My guess is that the rings aren’t needed in the rear since the diameter of the stem is pretty small, resulting in a small escape area.

I suspect the fouling you mention comes largely from the gas tube/key area.  Unless the rings are worn or the gaps are lined up, I’d think the rings would minimize gas leakage in that direction.

Also, keep in mind that when the bolt carrier moves to the rear, the carrier key and gas tube separate pretty quickly – I suspect residual gas pressure from the gas tube may blow fouling all over the place.

Just guessing, of course!

I have read a patent for a modified gas system from a gentleman named Michael Morris.  It modifies the carrier key so that it extends out of the upper for several inches (under the handguards).  The gas tube is shortened so that it does not enter the upper at all.  The extended carrier key also overlaps the gas tube enough that they never separate, and the gas tube always travels within the carrier key.  The gas tube is supported with a stand-off from the barrel just ahead of the end of the carrier key.  If the leakage between the gas tube and the carrier key is a major source of fouling in the upper this would seem to eliminate it. Any nastiness would be deposited in the space between the handguards and barrel where it would cause no problems either functionally or cosmetically.  Every once in a while you could wipe off the barrel and inside of the handguards if you got the urge.

A down side to this approach might be the required hole in the upper to allow the carrier key to extend outside.  This could allow debris into the upper.  The extended carrier key might be prone to bending and/or stressing the carrier key bolts because of the added weight and length.  But overall I like the approach.  Makes me wonder if it ever got off the ground.