Niklas;
Guess 2 1/2 is a designation, not a dimention.
It is my understanding the 2½" designation has been more or less simply a designation for a great many years. Both Greener & Burrard mention the chambers themselves going to 2 9/16" (65mm). According to Burrard after the introduction of the fold crimp in England in the late 30's all nominal 2½" shells so crimped have had a length of at least 67mm (2 5/8") with some being a true 2 3/4" (70mm). All were of course loaded properly for the "Nominal" 2½" guns.
