2.5" gun, chambers actually measure about 2.575". The first 16th or a bit more is more steeply angled(but not a step)than the rest of the forcing cone as shown in first pic. Second pic is of a Cheddite 2.735" paper hull with a card wad in the mouth. The long hulls(plastic too)will snug down into the chamber with very little finger pressure.
I do not think this has been messed with but I have never heard of this and cannot figure out the rationale unless this might have been an advantage with thick paper cases?
Any ideas?
Best,
Mark