That resilient bolt tail crud......
***Caveat to save the inevitable "You don't need to clean the bolt tail it's unnecessary/self-limiting/whatever" comments: I don't care what you do with your AR, I clean mine.
Well, haven't found it yet! I consider the bolt tail area the benchmark of baked-on AR carbon; no other part gets it as bad and as resistant to cleaners/solvents/CLP's. So far the "best" uber-lube I've heard of is FireClean. I dumped 1 1/2 bottles of it into an old pill bottle, enough to totally submerge a bolt as well as the front half (the dirty part) of a carrier. Not big enough to do both at the same time mind you. Got done shooting 400 rounds this past weekend and Sunday night I dropped the bolt into the FireClean after just wiping it off first. Let it sit until Monday afternoon and went to town. The gunk wiped off the rest of the bolt but lo and behold that damn bolt tail still required a little elbow grease and my green scratchy pad (and it's even a chromed bolt too!). Don't get me wrong, FireClean required LESS effort and time than other lubes but it still wasn't 100% on that particular area.
Now, I like the idea of FireClean and will continue to use it, but there is no silver bullet---to date---for the bolt tail crud. That shit is baked on there and physical energy will be required even with the best of cleaning lubes. I surmise that any substance that would actually dissolve the bolt tail crud would likely eat the metal away too!
Must state that I didn't "treat" it with FireClean before shooting (was using EWL) but in another thread someone said that a surface such as hard chrome will be limited in it's ability to absorb the FireClean. It's a much more "sealed" surface if you will as opposed to the more porous phosphate finish.