Couple additional points:
Prior to the advent of the plastic hull and wad, some American manufacturers intentionally short-chambered their guns so that the paper hulls would open into the forcing cone--the idea being that the hull mouth would give some protection to the shot on its initial contact with the barrel. Pattern tests did indeed show improvement with the use of slightly longer shells in short chambers.
As for Greener and Burrard, while the former did have something to sell (guns), I don't believe that Burrard did.
Re pressure increases from Bell's tests: All 4 of the increases to which I referred above--one of 1,000+ psi, the other 3 in the 900 psi range--came from 2 3/4" hulls fired in a test gun with a 2 1/2" chamber. The very hot 3" load Bell tested--near SAAMI max pressure--only resulted in an increase of about 700 psi when fired in the 2 1/2" chambered test gun.
