David,
According to my copy of the Birmingham Proof House Act of 1955, Appendix 3, table 2 a nominal bore size of "2" corrosponds to 1.325 inches for "barrels of arms of the first class and smooth bore arms of the fourth class".

FWIW they only give bore dimensions for bores "A" (2") through "3" (1.157"). From "4" down they give chamber dimensions.

The Act of 1925 shows a 2 bore load using 2350 grains of Powder, T.P. or 5oz., 6dr. load.. 1925 Act shows a bullet weight of 3342 grains.

The Act of 1868 Scale for Rifled Arms shows a 2 bore having a bore range of 1.250-1.325,

For proof,

a bullet diameter of 1.230-1.305,

bullet length of 1.845-1.958,

ratio of Length to diam. 1.500,

There are also powder charges and service charge listings.

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- Errol Flynn