To get back to Burrard, Thomas, etc on recoil and pressure:

Miller, I'm not sure that Thomas' theory as to WHY the faster burning powder creates less recoil is correct. However, even you state that you don't think he was a liar, and that the IMI tests to which he refers did happen, and the results were as reported. Did he report "complete" results, including types of powder, etc? Not anywhere I can find. However, if you're going to fault Thomas for that, then you also need to fault your hero Burrard, who only posted the CONCLUSION that long shells (appropriately loaded), when fired in short chambers, do not increase pressure. Thomas, on the other hand, provided an actual chart comparing 10 shots fired in a 2 1/2" chamber and 10 fired in a 2 3/4" chamber, with pressure readings taken at 1" and 6" from the breech, as well as velocity readings. (In this respect, his information is more complete than Bell's, although it only involves one type of shell.)

Burrard did indeed conclude, as I quoted, that excessive constriction of a shell would result in increased pressure. (P. 153, "The Modern Shotgun".) Bell essentially confirmed that in his tests, although in most cases the pressure increase was not very great.

And your recollection of the first "burst" produced in Bell's test is not exactly on target. What happened is that the chamber did indeed peel back where it's thickest, but the source of the weakness was not the solid chamber wall itself. Rather, it peeled back from the hole between the chambers that houses the ejectors. An obvious weak spot, if you stop to think about it.