Because the angle of the cone exceeds the angle of the external taper of the barrel, there is
more wall thickness within the cone than the end of the chamber.
The cone can be lengthened (
BY AN EXPERT, who is removing metal) while maintaining a wall thickness equal to the end of the chamber
Courtesy of Hollowell
Lengthened chamber with the blacked in portions being the metal removed in the process.
Upper side shows a 5 degree angle to the cone. Lower side shows the cone cut parallel with
the outer wall yielding a cone length of approximately 1 1/2".
Courtesy of Miller Fulks as posted on DoubleGunBBS
This is a cross-section of a chamber lengthened from 2 5/8" to slightly longer than 2 3/4".
This MAY NOT APPLY to small gauge or light weight British 2 1/2" chamber game guns.
Bottom line IMHO is a 12g No. 2 frame "shooter" Trojan with a 2 5/8" chamber could safely have the cone lengthened by an expert.
I'm not aware of any benefit in doing so.
I would
NOT mess with the chamber or cones of a short chamber 20g.