Robert;
The 20ga Flues I mentioned above had the chambers lengthened to 2 3/4". At the time I was using it was sort of in the transition era & it was shot with both paper & plastic shells. To the best of my recollection about the only shells I ever fed it were the 2½DE-1Oz loads, all in 2 3/4" hulls. I am not very familar with Gough Thomas' works so am unfamilar with the study you reference. Would like to here his reasoning on why plastic will not crack a frame but paper will. You can PM me if you like.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra