Ed,
I'm shooting 2 3/4" paper shells in my 2 9/16" chambered Parker and Ithaca guns. I also have a 2 9/16" chambered single shot with a pressure measurement device on which I checked pressures my load. I believe the Double Gun Journal article by Sherman Bell covered the issue on a 12ga pressure gun in which they cut a 2 1/2" short forcing cone chamber and measured pressures. My recollection was that they got from zero to +15% increase in pressures as compared to a modern chamber. I've seen at least this variation in pressure from loading manuals on recipes.