Generally Parker Bros. had the policy of holding chambers 1/8 inch shorter then the intended shell. Prior to the Great War, American ammo manufacturers offered 12-gauge paper shells in 2 5/8, 2 3/4, 2 7/8, 3, and 3 1/4 inch lengths; 16-gauge paper shells were offered in 2 9/16, 2 3/4, 2 7/8, and 3-inch lengths; and 20-gauge paper shells were offered in 2 1/2, 2 3/4, 2 7/8, and 3-inch lengths. Chamber lengths may also vary by what the customer asked for. Very likely if the customer asked that his gun be "chambered for 3-inch shells", the chambers would in fact be 2 7/8 inch. If the customer requested that his gun "have 3-inch chambers", very likely that would be what the Brothers P. provided.
My gut feeling is that the great bulk of the Parker 12-gauge doubles we see offered today with chambers longer then 2 5/8 inch were lengthened sometime during their life.
Also, chamber gauges are not all that accurate. If the chamber is slightly undersize in diameter the gauge won't go in all the way to the break for the forcing cone and give a false short reading. If the chamber is a bit oversize, the plug will go in past the break into the forcing cone and give a false long reading.
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