Most American 12-gauge doubles with 2 5/8 inch chambers were intended for 2 3/4 inch shells back in the day. That is certainly the case with Parker Bros. and A.H. Fox Gun Co. doubles. The belief was that the mouth of the case protruding a bit into the forcing cone cushioned the shot charge on its transition from shell to bore, and improved patterns.
There were a couple of articles by Arthur P. Curtis in The American Rifleman in 1936 and 1938 on the virtue of slightly short chambers. Testing by the British Proof house and tests by Sherman Bell, in his series "finding out for myself" which he reported in several issues of The Double Gun Journal, show no significant increase in pressure from firing shells in slightly short chambers.
I don't see any benefit in removing metal from a critical area of a gun barrel.
Chamber Lengthening
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