1"=25.4mm
76mm = 2.957" (3")
73mm = 2.874" (2 7/8")
70mm = 2.756" (2 3/4")
67mm = 2.637" (2 5/8")
65mm = 2.559" (2 9/16")
63.5mm = 2.500" (2½")
Many of these "Nominal" dimensions have been used rather loosly by various makers over the years.
For the most part I believe except for some "Very Early" guns most "nominal" 2½" British guns will in fact measure at 65mm or 2 9/16", some even to 2 5/8".
In the US many early 10 & 12ga's were 2 5/8", 16ga 2 9/16" & 20 & 28's 2½". Both 10 & 28 were standardized @ 2 7/8", 12 & 20 Ga @ 2 3/4" with 16ga being the last to go to the 2 3/4" standard. At some much later date the 28ga also dropped back to the 2 3/4" standard.
Other lengths were normally available to order.
Have not actually measured enough metric marked guns to say for ceretain, but I think they were much more apt to "Measure as Marked".
