I have thought, since the outset of this thread, that iron oxide could well be one of the grits that becomes embedded in the wood.
We use many different oxides for shaping and polishing metals. Some are in paste form and others are embedded onto paper or other surfaces. Many of them happen naturally in nature.
Time. Another variable methinks would be hard to replicate, were someone to come up with an alternative explanation of un-round muzzles is the time factor. The work you are doing Rich is critical for you to do in an expeditious manner--there are only so many hours in each day to move the material you're cutting, by cutter or compound. If it took you two weeks to choke a bore, you'd likely have to charge more--or in actuality likely not do the work in the first place.
How long is the total time of cleaning and loading (ramming and wiping) of an often shot ML over some 5 or 10 or 25 years of "hard use"? If someone would calculate up some numbers of strokes that might occur over those lengths of time, then a machine could be programmed (no pilgrim is going to volunteer--time) to apply those strokes and then we could have a "lab-grade" experiment. With that sort of setup a various compounds of fouling, iron-oxide, silt, sand, mud, could be tested as well as different woods and angles of deflection, pressure, etc.
Sure we could never test it all the variables out, but I still think that the amount of work we care to put into a bore by hand in one day or two, is a far cry from what might normally happen over some period of years, even if fully unintentionally.
Has not someone, somewhere saved a bbl cutoff? Perhaps microscopic examinations, or other tests, on such wobbled spouts could shed more insight as to their development. The fresh end would be right there to examine as well.
I fully trust they do exist, that in-fact bores have become misshapen over time, and a simple shortening has restored their accuracy. I'm only looking for alternative explanations for the premise here- that it cannot be done with wood and grit, but that there is another factor universally overlooked.