There are opinions, and there are facts
1. We have NO modern data as to the pressure rise of using long for chamber 20g shells. Bell's studies were 12g.
2. After some dumpster diving at the gun club, it was confirmed that modern 20g hulls are often shorter than 2 3/4"
3. Because the taper of the cones is sharper than the external taper of the barrel, forcing cones can be lengthened (metal removed) BY AN EXPERT resulting in a wall thickness no less than the end of the chamber, and by Boyle's law, pressure will be diminished. How much, no one knows
4. I can testify that using AA 7/8 oz target loads in a c. 1924 Parker 20g with chambers close to 2 3/8" yields
wicked recoil, presumable from the case mouth entering the cone
Opinion:
1. I would
not lengthen the chambers of a small bore, or lightweight English 12 bore game gun
2. Consider forcing cone lengthening by a barrel specialist like Dean Harris
3. Find a load that runs shorter than 2 3/4" and is 7/8 oz at 1200 fps or less.
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