As is usual, this discussion has confused powder fouling, which is relatively easy to remove with a bronze brush and solvent, with hard carbon that no solvent that I am aware of will touch. It also is complicated by the high probability that many of those that have responded may not have bore scopes.
As I sit here having reread the last of Donovan's posts, wondering why he found softer brushes ineffective with IOSSO, when I have (given that we both have bore scopes) the thought occurs to me that perhaps it is because of a difference in how the IOSSO is applied to the brush. The method that I use came to me from a account of how Tony Boyer would use it in which he said that he filled the entire brush with IOSSO, not just putting some on the tops of the bristles, but actually filling the brush from the core out to cover their tips, which takes quite a lot of material. A friend, who also has a bore scope, stores his IOSSO brush (for a particular caliber) in a plastic tube and does not remove the remaining IOSSO after he has used the brush for that purpose, in order not to have to use so much material. This works fine for him, and I may pick up a close fitting tube for that purpose. The most recent case of my using IOSSO for hard carbon was for a friends .20 caliber varmint rifle. It I worked the back of the barrel with short strokes, reversing the brush in the bore, and was able to remove the hard carbon without any difficulty. The barrel had been thouroughly cleaned using conventional materials and methods before hand, and inspected before and after I cleaned it with a bore scope,so I have no doubt about the results. In any case, having not used the harder brushes for this purpose, I cannot comment on any difference in their action, but I am pretty sure that IOSSO would not recommend anything that caused any sort of a problem. When Harder nylon brushes first came out, the thought was that they could be used instead of bronze brushes for general, solvent based bore maintenance. Even though I do not think that bronze brushes are a problem in that way, out of curiosity, I gave them a try, and found that for me, they were not as good as bronze, for use with liquid solvents. Others may have had different experiences. I believe that the brushes that I tried did not come from IOSSO (I believe that it was before they started offering them.) , but that has been a long time ago. One other variable in this is the amount of hard carbon that I normally have to deal with. Because I regularly inspect my barrels, and most of my shooting is with relatively clean powders in a 6PPC or small capacity varmint cartridges, my cleaning problems have probably not been as great as those who shoot different powders in larger cases, and have to deal with customers', rifles which may have a bigger problem because they have been neglected. Bottom line, I have no quarrel with anyone else's methods, but rather have only related mine, how they came to be, and the results that I have gotten. If I were starting fresh today, I would undoubtedly have simply read the manufacturer's instructions and followed them.