Please excuse me if this article has already been mentioned before.  In Vol. 22, issue 2 (Summer 2011) of The Double Gun Journal I have just been reading "Finding Out for Myself, Part XVII, Express Rifle Case Fillers" by Sherman Bell.  He used a pressure barrel to record results of using numerous fillers on top of 48 gr. of 4198 in the .450, 3 1/4" black-powder case.  Particularly Bell was interested in the pressure at the base of the bullet where ringing might occur.  Some surprising conclusions.

I hate to paraphrase the large amount of information in this article.  To quote the author, "Those hoping for clear-cut heroes and villains to surface will be disappointed."  None of the 15 loading methods produced high spikes of pressure or ringing.  The test barrel was modern steel, however.  The author noted smooth pressure curves from case-filling amounts of Dacron, Kapok, Cotton, and felt wads.  Even good old Cream of Wheat performed well with just slightly higher pressure.  Read the article.