Hatcher mentioned that there was a sort of fad between the wars wherein every undiagnosed problem was blamed on headspace as a matter or routine. It didn't matter what the actual cause was. If it was mysterious, it was a headspace problem. I think the same thing may have developed regarding gas rings and the bolt carriers for the AR platform? Your description makes it sound like they replaced the bolt carrier and maybe the rings? The carrier could have been defective, I suppose, but without having it in hand to gauge, we'll never know? If the guy who fixed the upper found some embarrassing problem with the gas system that should not have got past inspection, you will never hear about it.
Old Grump gave you a version of a method that is useful for a number of things. I mix up Ed's Red, just to have a formula, and keep it in a gallon paint can. Immediately after shooting, I dip the carrier group in and run a wet patch of it through the bore. Same with a 1911 slide and barrel.
Carbon hardens over time. If you get a carbon softening solvent on a carbon deposit before it gets hard it is a lot easier to clean off later. There is a long diatribe on this topic by a former Aberdeen Proving Grounds employee, in
post 25, here. A commercial preparation that will soften even hardened carbon overnight is Gunzilla.