Sherman Bell's testing was very interesting and covered the gamut of filler material. He said he was surprised that the closed cell foam which he knew of to have caused 2 instances of chamber ringing did not show any reason in his test to have done so. What he did not test was any powder except 4198. He did not mention if the two instances of ringing involved 4198 or some other powder or if those instances were in older barrels which he admitted may be at more risk than the modern barrel material he was using. I would be mindful of the tests that produced "stepping" of the pressure curves as the more likely problem makers under slightly different conditions.
