Interestingly enough, prior to the advent of plastic shells and wads, there were quite a few "experts" who advocated the use of longer hulls in short chambers. The result, as evidenced in various tests, was improved patterns. Reason: the paper hull, opening into the cone, provided some protection for the shot on its initial contact with the barrel (since it was not contained in a plastic wad).
From my own experience, which is fairly extensive in terms of firing 2 3/4" standard crimped shells in both 12ga and 16ga guns with short chambers and "unbutchered" cones, I can't say I've noticed that the crimp is any more "partially closed" than it is when fired in a 2 3/4" chamber. And the author of the article on long shells in short chambers in "The American Rifleman" back in the 30's showed graphic photographic evidence that 2 3/4" paper shells, fired in 2 1/2" chambers, did indeed show evidence of contact with the shot charge: "Viewing, in Figure 2, those shells that have been shot in short chambers and as a consequence are tapered on the ends and have the paper cut by the shot to a thin edge . . . " A.P. Curtis, "Advantages of Short Shotgun Chambers", The American Rifleman, July 1936. Curtis thought this an advantage, as mentioned above. But it does not exactly leave the shell "open flat".
For several years, I had a pair of British 12ga boxlocks with unaltered 2 1/2" chambers and forcing cones. I shot thousands of 2 3/4" shells (loaded to appropriately low pressure) through those guns. Cleaning them revealed no evidence of excess plastic fouling. Bores came out nice and shiny with the same combination of Hoppe's 9 followed by a light coat of gun oil that I used on my modern guns.
