Bell's 12 gauge study showed an average increase in pressure of 700 psi using 2 3/4" shells in 2 1/2" chambers. The increase in one load was 1200 psi.
He also demonstrated the lengthening forcing cones, without lengthening the chamber, does lessen that pressure increase somewhat.

It would be prudent (and easy) to check for 'feathering' of the case mouth in spent shells, and if so the mouth has entered the forcing cone.
67 mm Left; 65 mm on Right

[Linked Image from photos.smugmug.com]

It has also been observed that nominally 2 3/4" shells after firing are often shorter

[Linked Image from photos.smugmug.com]

And, again, WE HAVE NO DATA FOR USING LONG FOR CHAMBER 16G AND 20G SHELLS

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